root/galaxy-central/eggs/SQLAlchemy-0.5.6_dev_r6498-py2.6.egg/sqlalchemy/sql/expression.py

# expression.py
# Copyright (C) 2005, 2006, 2007, 2008, 2009 Michael Bayer mike_mp@zzzcomputing.com
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

"""Defines the base components of SQL expression trees.

All components are derived from a common base class
:class:`~sqlalchemy.sql.expression.ClauseElement`.  Common behaviors are organized
based on class hierarchies, in some cases via mixins.

All object construction from this package occurs via functions which
in some cases will construct composite ``ClauseElement`` structures
together, and in other cases simply return a single ``ClauseElement``
constructed directly.  The function interface affords a more "DSL-ish"
feel to constructing SQL expressions and also allows future class
reorganizations.

Even though classes are not constructed directly from the outside,
most classes which have additional public methods are considered to be
public (i.e. have no leading underscore).  Other classes which are
"semi-public" are marked with a single leading underscore; these
classes usually have few or no public methods and are less guaranteed
to stay the same in future releases.

"""

import itertools, re
from operator import attrgetter

from sqlalchemy import util, exc
from sqlalchemy.sql import operators
from sqlalchemy.sql.visitors import Visitable, cloned_traverse
from sqlalchemy import types as sqltypes
import operator

functions, schema, sql_util = None, None, None
DefaultDialect, ClauseAdapter, Annotated = None, None, None

__all__ = [
    'Alias', 'ClauseElement',
    'ColumnCollection', 'ColumnElement',
    'CompoundSelect', 'Delete', 'FromClause', 'Insert', 'Join',
    'Select', 'Selectable', 'TableClause', 'Update', 'alias', 'and_', 'asc',
    'between', 'bindparam', 'case', 'cast', 'column', 'delete',
    'desc', 'distinct', 'except_', 'except_all', 'exists', 'extract', 'func',
    'modifier', 'collate',
    'insert', 'intersect', 'intersect_all', 'join', 'label', 'literal',
    'literal_column', 'not_', 'null', 'or_', 'outparam', 'outerjoin', 'select',
    'subquery', 'table', 'text', 'union', 'union_all', 'update', ]



def desc(column):
    """Return a descending ``ORDER BY`` clause element.

    e.g.::

      order_by = [desc(table1.mycol)]

    """
    return _UnaryExpression(column, modifier=operators.desc_op)

def asc(column):
    """Return an ascending ``ORDER BY`` clause element.

    e.g.::

      order_by = [asc(table1.mycol)]

    """
    return _UnaryExpression(column, modifier=operators.asc_op)

def outerjoin(left, right, onclause=None):
    """Return an ``OUTER JOIN`` clause element.

    The returned object is an instance of :class:`~sqlalchemy.sql.expression.Join`.

    Similar functionality is also available via the ``outerjoin()``
    method on any :class:`~sqlalchemy.sql.expression.FromClause`.

    left
      The left side of the join.

    right
      The right side of the join.

    onclause
      Optional criterion for the ``ON`` clause, is derived from
      foreign key relationships established between left and right
      otherwise.

    To chain joins together, use the ``join()`` or ``outerjoin()``
    methods on the resulting ``Join`` object.

    """
    return Join(left, right, onclause, isouter=True)

def join(left, right, onclause=None, isouter=False):
    """Return a ``JOIN`` clause element (regular inner join).

    The returned object is an instance of :class:`~sqlalchemy.sql.expression.Join`.

    Similar functionality is also available via the ``join()`` method
    on any :class:`~sqlalchemy.sql.expression.FromClause`.

    left
      The left side of the join.

    right
      The right side of the join.

    onclause
      Optional criterion for the ``ON`` clause, is derived from
      foreign key relationships established between left and right
      otherwise.

    To chain joins together, use the ``join()`` or ``outerjoin()``
    methods on the resulting ``Join`` object.

    """
    return Join(left, right, onclause, isouter)

def select(columns=None, whereclause=None, from_obj=[], **kwargs):
    """Returns a ``SELECT`` clause element.

    Similar functionality is also available via the ``select()``
    method on any :class:`~sqlalchemy.sql.expression.FromClause`.

    The returned object is an instance of :class:`~sqlalchemy.sql.expression.Select`.

    All arguments which accept ``ClauseElement`` arguments also accept
    string arguments, which will be converted as appropriate into
    either ``text()`` or ``literal_column()`` constructs.

    columns
      A list of ``ClauseElement`` objects, typically ``ColumnElement``
      objects or subclasses, which will form the columns clause of the
      resulting statement.  For all members which are instances of
      ``Selectable``, the individual ``ColumnElement`` members of the
      ``Selectable`` will be added individually to the columns clause.
      For example, specifying a ``Table`` instance will result in all
      the contained ``Column`` objects within to be added to the
      columns clause.

      This argument is not present on the form of ``select()``
      available on ``Table``.

    whereclause
      A ``ClauseElement`` expression which will be used to form the
      ``WHERE`` clause.

    from_obj
      A list of ``ClauseElement`` objects which will be added to the
      ``FROM`` clause of the resulting statement.  Note that "from"
      objects are automatically located within the columns and
      whereclause ClauseElements.  Use this parameter to explicitly
      specify "from" objects which are not automatically locatable.
      This could include ``Table`` objects that aren't otherwise
      present, or ``Join`` objects whose presence will supercede that
      of the ``Table`` objects already located in the other clauses.

    \**kwargs
      Additional parameters include:

      autocommit
        indicates this SELECT statement modifies the database, and
        should be subject to autocommit behavior if no transaction
        has been started.

      prefixes
        a list of strings or ``ClauseElement`` objects to include
        directly after the SELECT keyword in the generated statement,
        for dialect-specific query features.

      distinct=False
        when ``True``, applies a ``DISTINCT`` qualifier to the columns
        clause of the resulting statement.

      use_labels=False
        when ``True``, the statement will be generated using labels
        for each column in the columns clause, which qualify each
        column with its parent table's (or aliases) name so that name
        conflicts between columns in different tables don't occur.
        The format of the label is <tablename>_<column>.  The "c"
        collection of the resulting ``Select`` object will use these
        names as well for targeting column members.

      for_update=False
        when ``True``, applies ``FOR UPDATE`` to the end of the
        resulting statement.  Certain database dialects also support
        alternate values for this parameter, for example mysql
        supports "read" which translates to ``LOCK IN SHARE MODE``,
        and oracle supports "nowait" which translates to ``FOR UPDATE
        NOWAIT``.

      correlate=True
        indicates that this ``Select`` object should have its
        contained ``FromClause`` elements "correlated" to an enclosing
        ``Select`` object.  This means that any ``ClauseElement``
        instance within the "froms" collection of this ``Select``
        which is also present in the "froms" collection of an
        enclosing select will not be rendered in the ``FROM`` clause
        of this select statement.

      group_by
        a list of ``ClauseElement`` objects which will comprise the
        ``GROUP BY`` clause of the resulting select.

      having
        a ``ClauseElement`` that will comprise the ``HAVING`` clause
        of the resulting select when ``GROUP BY`` is used.

      order_by
        a scalar or list of ``ClauseElement`` objects which will
        comprise the ``ORDER BY`` clause of the resulting select.

      limit=None
        a numerical value which usually compiles to a ``LIMIT``
        expression in the resulting select.  Databases that don't
        support ``LIMIT`` will attempt to provide similar
        functionality.

      offset=None
        a numeric value which usually compiles to an ``OFFSET``
        expression in the resulting select.  Databases that don't
        support ``OFFSET`` will attempt to provide similar
        functionality.

      bind=None
        an ``Engine`` or ``Connection`` instance to which the
        resulting ``Select ` object will be bound.  The ``Select``
        object will otherwise automatically bind to whatever
        ``Connectable`` instances can be located within its contained
        ``ClauseElement`` members.

      scalar=False
        deprecated.  Use select(...).as_scalar() to create a "scalar
        column" proxy for an existing Select object.

    """
    if 'scalar' in kwargs:
        util.warn_deprecated('scalar option is deprecated; see docs for details')
    scalar = kwargs.pop('scalar', False)
    s = Select(columns, whereclause=whereclause, from_obj=from_obj, **kwargs)
    if scalar:
        return s.as_scalar()
    else:
        return s

def subquery(alias, *args, **kwargs):
    """Return an :class:`~sqlalchemy.sql.expression.Alias` object derived from a :class:`~sqlalchemy.sql.expression.Select`.

    name
      alias name

    \*args, \**kwargs

      all other arguments are delivered to the :func:`~sqlalchemy.sql.expression.select`
      function.

    """
    return Select(*args, **kwargs).alias(alias)

def insert(table, values=None, inline=False, **kwargs):
    """Return an :class:`~sqlalchemy.sql.expression.Insert` clause element.

    Similar functionality is available via the ``insert()`` method on
    :class:`~sqlalchemy.schema.Table`.

    :param table: The table to be inserted into.

    :param values: A dictionary which specifies the column specifications of the
      ``INSERT``, and is optional.  If left as None, the column
      specifications are determined from the bind parameters used
      during the compile phase of the ``INSERT`` statement.  If the
      bind parameters also are None during the compile phase, then the
      column specifications will be generated from the full list of
      table columns.  Note that the :meth:`~Insert.values()` generative method
      may also be used for this.

    :param prefixes: A list of modifier keywords to be inserted between INSERT and INTO.
      Alternatively, the :meth:`~Insert.prefix_with` generative method may be used.

    :param inline:
      if True, SQL defaults will be compiled 'inline' into the statement
      and not pre-executed.

    If both `values` and compile-time bind parameters are present, the
    compile-time bind parameters override the information specified
    within `values` on a per-key basis.

    The keys within `values` can be either ``Column`` objects or their
    string identifiers.  Each key may reference one of:

    * a literal data value (i.e. string, number, etc.);
    * a Column object;
    * a SELECT statement.

    If a ``SELECT`` statement is specified which references this
    ``INSERT`` statement's table, the statement will be correlated
    against the ``INSERT`` statement.

    """
    return Insert(table, values, inline=inline, **kwargs)

def update(table, whereclause=None, values=None, inline=False, **kwargs):
    """Return an :class:`~sqlalchemy.sql.expression.Update` clause element.

    Similar functionality is available via the ``update()`` method on
    :class:`~sqlalchemy.schema.Table`.

    :param table: The table to be updated.

    :param whereclause: A ``ClauseElement`` describing the ``WHERE`` condition of the
      ``UPDATE`` statement.  Note that the :meth:`~Update.where()` generative
      method may also be used for this.

    :param values:
      A dictionary which specifies the ``SET`` conditions of the
      ``UPDATE``, and is optional. If left as None, the ``SET``
      conditions are determined from the bind parameters used during
      the compile phase of the ``UPDATE`` statement.  If the bind
      parameters also are None during the compile phase, then the
      ``SET`` conditions will be generated from the full list of table
      columns.  Note that the :meth:`~Update.values()` generative method may
      also be used for this.

    :param inline:
      if True, SQL defaults will be compiled 'inline' into the statement
      and not pre-executed.

    If both `values` and compile-time bind parameters are present, the
    compile-time bind parameters override the information specified
    within `values` on a per-key basis.

    The keys within `values` can be either ``Column`` objects or their
    string identifiers. Each key may reference one of:

    * a literal data value (i.e. string, number, etc.);
    * a Column object;
    * a SELECT statement.

    If a ``SELECT`` statement is specified which references this
    ``UPDATE`` statement's table, the statement will be correlated
    against the ``UPDATE`` statement.

    """
    return Update(table, whereclause=whereclause, values=values, inline=inline, **kwargs)

def delete(table, whereclause = None, **kwargs):
    """Return a :class:`~sqlalchemy.sql.expression.Delete` clause element.

    Similar functionality is available via the ``delete()`` method on
    :class:`~sqlalchemy.schema.Table`.

    :param table: The table to be updated.

    :param whereclause: A :class:`ClauseElement` describing the ``WHERE`` condition of the
      ``UPDATE`` statement.  Note that the :meth:`~Delete.where()` generative method
      may be used instead.

    """
    return Delete(table, whereclause, **kwargs)

def and_(*clauses):
    """Join a list of clauses together using the ``AND`` operator.

    The ``&`` operator is also overloaded on all
    :class:`~sqlalchemy.sql.expression._CompareMixin` subclasses to produce the same
    result.

    """
    if len(clauses) == 1:
        return clauses[0]
    return BooleanClauseList(operator=operators.and_, *clauses)

def or_(*clauses):
    """Join a list of clauses together using the ``OR`` operator.

    The ``|`` operator is also overloaded on all
    :class:`~sqlalchemy.sql.expression._CompareMixin` subclasses to produce the same
    result.

    """
    if len(clauses) == 1:
        return clauses[0]
    return BooleanClauseList(operator=operators.or_, *clauses)

def not_(clause):
    """Return a negation of the given clause, i.e. ``NOT(clause)``.

    The ``~`` operator is also overloaded on all
    :class:`~sqlalchemy.sql.expression._CompareMixin` subclasses to produce the same
    result.

    """
    return operators.inv(_literal_as_binds(clause))

def distinct(expr):
    """Return a ``DISTINCT`` clause."""
    expr = _literal_as_binds(expr)
    return _UnaryExpression(expr, operator=operators.distinct_op, type_=expr.type)

def between(ctest, cleft, cright):
    """Return a ``BETWEEN`` predicate clause.

    Equivalent of SQL ``clausetest BETWEEN clauseleft AND clauseright``.

    The ``between()`` method on all :class:`~sqlalchemy.sql.expression._CompareMixin` subclasses
    provides similar functionality.

    """
    ctest = _literal_as_binds(ctest)
    return _BinaryExpression(
        ctest,
        ClauseList(
            _literal_as_binds(cleft, type_=ctest.type),
            _literal_as_binds(cright, type_=ctest.type),
            operator=operators.and_,
            group=False),
        operators.between_op)


def case(whens, value=None, else_=None):
    """Produce a ``CASE`` statement.

    whens
      A sequence of pairs, or alternatively a dict,
      to be translated into "WHEN / THEN" clauses.

    value
      Optional for simple case statements, produces
      a column expression as in "CASE <expr> WHEN ..."

    else\_
      Optional as well, for case defaults produces
      the "ELSE" portion of the "CASE" statement.

    The expressions used for THEN and ELSE,
    when specified as strings, will be interpreted
    as bound values. To specify textual SQL expressions
    for these, use the text(<string>) construct.

    The expressions used for the WHEN criterion
    may only be literal strings when "value" is
    present, i.e. CASE table.somecol WHEN "x" THEN "y".
    Otherwise, literal strings are not accepted
    in this position, and either the text(<string>)
    or literal(<string>) constructs must be used to
    interpret raw string values.

    Usage examples::

      case([(orderline.c.qty > 100, item.c.specialprice),
            (orderline.c.qty > 10, item.c.bulkprice)
          ], else_=item.c.regularprice)
      case(value=emp.c.type, whens={
              'engineer': emp.c.salary * 1.1,
              'manager':  emp.c.salary * 3,
          })

    """
    
    return _Case(whens, value=value, else_=else_)

def cast(clause, totype, **kwargs):
    """Return a ``CAST`` function.

    Equivalent of SQL ``CAST(clause AS totype)``.

    Use with a :class:`~sqlalchemy.types.TypeEngine` subclass, i.e::

      cast(table.c.unit_price * table.c.qty, Numeric(10,4))

    or::

      cast(table.c.timestamp, DATE)

    """
    return _Cast(clause, totype, **kwargs)

def extract(field, expr):
    """Return the clause ``extract(field FROM expr)``."""

    return _Extract(field, expr)

def collate(expression, collation):
    """Return the clause ``expression COLLATE collation``."""

    expr = _literal_as_binds(expression)
    return _BinaryExpression(
        expr, 
        _literal_as_text(collation), 
        operators.collate, type_=expr.type)

def exists(*args, **kwargs):
    """Return an ``EXISTS`` clause as applied to a :class:`~sqlalchemy.sql.expression.Select` object.

    Calling styles are of the following forms::

        # use on an existing select()
        s = select([table.c.col1]).where(table.c.col2==5)
        s = exists(s)

        # construct a select() at once
        exists(['*'], **select_arguments).where(criterion)

        # columns argument is optional, generates "EXISTS (SELECT *)"
        # by default.
        exists().where(table.c.col2==5)

    """
    return _Exists(*args, **kwargs)

def union(*selects, **kwargs):
    """Return a ``UNION`` of multiple selectables.

    The returned object is an instance of :class:`~sqlalchemy.sql.expression.CompoundSelect`.

    A similar ``union()`` method is available on all
    :class:`~sqlalchemy.sql.expression.FromClause` subclasses.

    \*selects
      a list of :class:`~sqlalchemy.sql.expression.Select` instances.

    \**kwargs
       available keyword arguments are the same as those of
       :func:`~sqlalchemy.sql.expression.select`.

    """
    return _compound_select('UNION', *selects, **kwargs)

def union_all(*selects, **kwargs):
    """Return a ``UNION ALL`` of multiple selectables.

    The returned object is an instance of :class:`~sqlalchemy.sql.expression.CompoundSelect`.

    A similar ``union_all()`` method is available on all
    :class:`~sqlalchemy.sql.expression.FromClause` subclasses.

    \*selects
      a list of :class:`~sqlalchemy.sql.expression.Select` instances.

    \**kwargs
      available keyword arguments are the same as those of
      :func:`~sqlalchemy.sql.expression.select`.

    """
    return _compound_select('UNION ALL', *selects, **kwargs)

def except_(*selects, **kwargs):
    """Return an ``EXCEPT`` of multiple selectables.

    The returned object is an instance of :class:`~sqlalchemy.sql.expression.CompoundSelect`.

    \*selects
      a list of :class:`~sqlalchemy.sql.expression.Select` instances.

    \**kwargs
      available keyword arguments are the same as those of
      :func:`~sqlalchemy.sql.expression.select`.

    """
    return _compound_select('EXCEPT', *selects, **kwargs)

def except_all(*selects, **kwargs):
    """Return an ``EXCEPT ALL`` of multiple selectables.

    The returned object is an instance of :class:`~sqlalchemy.sql.expression.CompoundSelect`.

    \*selects
      a list of :class:`~sqlalchemy.sql.expression.Select` instances.

    \**kwargs
      available keyword arguments are the same as those of
      :func:`~sqlalchemy.sql.expression.select`.

    """
    return _compound_select('EXCEPT ALL', *selects, **kwargs)

def intersect(*selects, **kwargs):
    """Return an ``INTERSECT`` of multiple selectables.

    The returned object is an instance of :class:`~sqlalchemy.sql.expression.CompoundSelect`.

    \*selects
      a list of :class:`~sqlalchemy.sql.expression.Select` instances.

    \**kwargs
      available keyword arguments are the same as those of
      :func:`~sqlalchemy.sql.expression.select`.

    """
    return _compound_select('INTERSECT', *selects, **kwargs)

def intersect_all(*selects, **kwargs):
    """Return an ``INTERSECT ALL`` of multiple selectables.

    The returned object is an instance of :class:`~sqlalchemy.sql.expression.CompoundSelect`.

    \*selects
      a list of :class:`~sqlalchemy.sql.expression.Select` instances.

    \**kwargs
      available keyword arguments are the same as those of
      :func:`~sqlalchemy.sql.expression.select`.

    """
    return _compound_select('INTERSECT ALL', *selects, **kwargs)

def alias(selectable, alias=None):
    """Return an :class:`~sqlalchemy.sql.expression.Alias` object.

    An ``Alias`` represents any :class:`~sqlalchemy.sql.expression.FromClause` with
    an alternate name assigned within SQL, typically using the ``AS``
    clause when generated, e.g. ``SELECT * FROM table AS aliasname``.

    Similar functionality is available via the ``alias()`` method
    available on all ``FromClause`` subclasses.

      selectable
        any ``FromClause`` subclass, such as a table, select
        statement, etc..

      alias
        string name to be assigned as the alias.  If ``None``, a
        random name will be generated.

    """
    return Alias(selectable, alias=alias)


def literal(value, type_=None):
    """Return a literal clause, bound to a bind parameter.

    Literal clauses are created automatically when non-
    ``ClauseElement`` objects (such as strings, ints, dates, etc.) are
    used in a comparison operation with a
    :class:`~sqlalchemy.sql.expression._CompareMixin` subclass, such as a ``Column``
    object.  Use this function to force the generation of a literal
    clause, which will be created as a
    :class:`~sqlalchemy.sql.expression._BindParamClause` with a bound value.

    value
      the value to be bound.  Can be any Python object supported by
      the underlying DB-API, or is translatable via the given type
      argument.

    type\_
      an optional :class:`~sqlalchemy.types.TypeEngine` which will provide
      bind-parameter translation for this literal.

    """
    return _BindParamClause(None, value, type_=type_, unique=True)

def label(name, obj):
    """Return a :class:`~sqlalchemy.sql.expression._Label` object for the given :class:`~sqlalchemy.sql.expression.ColumnElement`.

    A label changes the name of an element in the columns clause of a
    ``SELECT`` statement, typically via the ``AS`` SQL keyword.

    This functionality is more conveniently available via the
    ``label()`` method on ``ColumnElement``.

    name
      label name

    obj
      a ``ColumnElement``.

    """
    return _Label(name, obj)

def column(text, type_=None):
    """Return a textual column clause, as would be in the columns clause of a ``SELECT`` statement.

    The object returned is an instance of :class:`~sqlalchemy.sql.expression.ColumnClause`,
    which represents the "syntactical" portion of the schema-level
    :class:`~sqlalchemy.schema.Column` object.

    text
      the name of the column.  Quoting rules will be applied to the
      clause like any other column name.  For textual column
      constructs that are not to be quoted, use the
      :func:`~sqlalchemy.sql.expression.literal_column` function.

    type\_
      an optional :class:`~sqlalchemy.types.TypeEngine` object which will
      provide result-set translation for this column.

    """
    return ColumnClause(text, type_=type_)

def literal_column(text, type_=None):
    """Return a textual column expression, as would be in the columns
    clause of a ``SELECT`` statement.

    The object returned supports further expressions in the same way as any
    other column object, including comparison, math and string operations.
    The type\_ parameter is important to determine proper expression behavior
    (such as, '+' means string concatenation or numerical addition based on
    the type).

    text
      the text of the expression; can be any SQL expression.  Quoting rules
      will not be applied.  To specify a column-name expression which should
      be subject to quoting rules, use the
      :func:`~sqlalchemy.sql.expression.column` function.

    type\_
      an optional :class:`~sqlalchemy.types.TypeEngine` object which will provide
      result-set translation and additional expression semantics for this
      column.  If left as None the type will be NullType.

    """
    return ColumnClause(text, type_=type_, is_literal=True)

def table(name, *columns):
    """Return a :class:`~sqlalchemy.sql.expression.TableClause` object.

    This is a primitive version of the :class:`~sqlalchemy.schema.Table` object,
    which is a subclass of this object.

    """
    return TableClause(name, *columns)

def bindparam(key, value=None, shortname=None, type_=None, unique=False):
    """Create a bind parameter clause with the given key.

    value
      a default value for this bind parameter.  a bindparam with a
      value is called a ``value-based bindparam``.

    type\_
      a sqlalchemy.types.TypeEngine object indicating the type of this
      bind param, will invoke type-specific bind parameter processing

    shortname
      deprecated.

    unique
      if True, bind params sharing the same name will have their
      underlying ``key`` modified to a uniquely generated name.
      mostly useful with value-based bind params.

    """
    if isinstance(key, ColumnClause):
        return _BindParamClause(key.name, value, type_=key.type, unique=unique, shortname=shortname)
    else:
        return _BindParamClause(key, value, type_=type_, unique=unique, shortname=shortname)

def outparam(key, type_=None):
    """Create an 'OUT' parameter for usage in functions (stored procedures), for databases which support them.

    The ``outparam`` can be used like a regular function parameter.
    The "output" value will be available from the
    :class:`~sqlalchemy.engine.ResultProxy` object via its ``out_parameters``
    attribute, which returns a dictionary containing the values.

    """
    return _BindParamClause(key, None, type_=type_, unique=False, isoutparam=True)

def text(text, bind=None, *args, **kwargs):
    """Create literal text to be inserted into a query.

    When constructing a query from a ``select()``, ``update()``,
    ``insert()`` or ``delete()``, using plain strings for argument
    values will usually result in text objects being created
    automatically.  Use this function when creating textual clauses
    outside of other ``ClauseElement`` objects, or optionally wherever
    plain text is to be used.

    text
      the text of the SQL statement to be created.  use ``:<param>``
      to specify bind parameters; they will be compiled to their
      engine-specific format.

    bind
      an optional connection or engine to be used for this text query.

    autocommit=True
      indicates this SELECT statement modifies the database, and
      should be subject to autocommit behavior if no transaction
      has been started.

    bindparams
      a list of ``bindparam()`` instances which can be used to define
      the types and/or initial values for the bind parameters within
      the textual statement; the keynames of the bindparams must match
      those within the text of the statement.  The types will be used
      for pre-processing on bind values.

    typemap
      a dictionary mapping the names of columns represented in the
      ``SELECT`` clause of the textual statement to type objects,
      which will be used to perform post-processing on columns within
      the result set (for textual statements that produce result
      sets).

    """
    return _TextClause(text, bind=bind, *args, **kwargs)

def null():
    """Return a :class:`_Null` object, which compiles to ``NULL`` in a sql statement."""

    return _Null()

class _FunctionGenerator(object):
    """Generate :class:`Function` objects based on getattr calls."""

    def __init__(self, **opts):
        self.__names = []
        self.opts = opts

    def __getattr__(self, name):
        # passthru __ attributes; fixes pydoc
        if name.startswith('__'):
            try:
                return self.__dict__[name]
            except KeyError:
                raise AttributeError(name)

        elif name.endswith('_'):
            name = name[0:-1]
        f = _FunctionGenerator(**self.opts)
        f.__names = list(self.__names) + [name]
        return f

    def __call__(self, *c, **kwargs):
        o = self.opts.copy()
        o.update(kwargs)
        if len(self.__names) == 1:
            global functions
            if functions is None:
                from sqlalchemy.sql import functions
            func = getattr(functions, self.__names[-1].lower(), None)
            if func is not None:
                return func(*c, **o)

        return Function(self.__names[-1], packagenames=self.__names[0:-1], *c, **o)

# "func" global - i.e. func.count()
func = _FunctionGenerator()

# "modifier" global - i.e. modifier.distinct
# TODO: use UnaryExpression for this instead ?
modifier = _FunctionGenerator(group=False)

class _generated_label(unicode):
    """A unicode subclass used to identify dynamically generated names."""

def _escape_for_generated(x):
    if isinstance(x, _generated_label):
        return x
    else:
        return x.replace('%', '%%')
        
def _clone(element):
    return element._clone()

def _expand_cloned(elements):
    """expand the given set of ClauseElements to be the set of all 'cloned' predecessors."""

    return itertools.chain(*[x._cloned_set for x in elements])

def _cloned_intersection(a, b):
    """return the intersection of sets a and b, counting
    any overlap between 'cloned' predecessors.

    The returned set is in terms of the enties present within 'a'.

    """
    all_overlap = set(_expand_cloned(a)).intersection(_expand_cloned(b))
    return set(elem for elem in a if all_overlap.intersection(elem._cloned_set))

def _compound_select(keyword, *selects, **kwargs):
    return CompoundSelect(keyword, *selects, **kwargs)

def _is_literal(element):
    return not isinstance(element, Visitable) and not hasattr(element, '__clause_element__')

def _from_objects(*elements):
    return itertools.chain(*[element._from_objects for element in elements])

def _labeled(element):
    if not hasattr(element, 'name'):
        return element.label(None)
    else:
        return element

def _column_as_key(element):
    if isinstance(element, basestring):
        return element
    if hasattr(element, '__clause_element__'):
        element = element.__clause_element__()
    return element.key
    
def _literal_as_text(element):
    if hasattr(element, '__clause_element__'):
        return element.__clause_element__()
    elif not isinstance(element, Visitable):
        return _TextClause(unicode(element))
    else:
        return element

def _clause_element_as_expr(element):
    if hasattr(element, '__clause_element__'):
        return element.__clause_element__()
    else:
        return element
        
def _literal_as_column(element):
    if hasattr(element, '__clause_element__'):
        return element.__clause_element__()
    elif not isinstance(element, Visitable):
        return literal_column(str(element))
    else:
        return element

def _literal_as_binds(element, name=None, type_=None):
    if hasattr(element, '__clause_element__'):
        return element.__clause_element__()
    elif not isinstance(element, Visitable):
        if element is None:
            return null()
        else:
            return _BindParamClause(name, element, type_=type_, unique=True)
    else:
        return element

def _no_literals(element):
    if hasattr(element, '__clause_element__'):
        return element.__clause_element__()
    elif not isinstance(element, Visitable):
        raise exc.ArgumentError("Ambiguous literal: %r.  Use the 'text()' function "
                "to indicate a SQL expression literal, or 'literal()' to indicate a bound value." % element)
    else:
        return element

def _corresponding_column_or_error(fromclause, column, require_embedded=False):
    c = fromclause.corresponding_column(column, require_embedded=require_embedded)
    if not c:
        raise exc.InvalidRequestError("Given column '%s', attached to table '%s', "
                "failed to locate a corresponding column from table '%s'"
                % (column, getattr(column, 'table', None), fromclause.description))
    return c

def is_column(col):
    """True if ``col`` is an instance of ``ColumnElement``."""
    return isinstance(col, ColumnElement)


class ClauseElement(Visitable):
    """Base class for elements of a programmatically constructed SQL expression."""

    __visit_name__ = 'clause'

    _annotations = {}
    supports_execution = False
    _from_objects = []

    def _clone(self):
        """Create a shallow copy of this ClauseElement.

        This method may be used by a generative API.  Its also used as
        part of the "deep" copy afforded by a traversal that combines
        the _copy_internals() method.

        """
        c = self.__class__.__new__(self.__class__)
        c.__dict__ = self.__dict__.copy()
        c.__dict__.pop('_cloned_set', None)

        # this is a marker that helps to "equate" clauses to each other
        # when a Select returns its list of FROM clauses.  the cloning
        # process leaves around a lot of remnants of the previous clause
        # typically in the form of column expressions still attached to the
        # old table.
        c._is_clone_of = self

        return c

    @util.memoized_property
    def _cloned_set(self):
        """Return the set consisting all cloned anscestors of this ClauseElement.

        Includes this ClauseElement.  This accessor tends to be used for
        FromClause objects to identify 'equivalent' FROM clauses, regardless
        of transformative operations.

        """
        s = util.column_set()
        f = self
        while f is not None:
            s.add(f)
            f = getattr(f, '_is_clone_of', None)
        return s

    def __getstate__(self):
        d = self.__dict__.copy()
        d.pop('_is_clone_of', None)
        return d

    def _annotate(self, values):
        """return a copy of this ClauseElement with the given annotations dictionary."""

        global Annotated
        if Annotated is None:
            from sqlalchemy.sql.util import Annotated
        return Annotated(self, values)

    def _deannotate(self):
        """return a copy of this ClauseElement with an empty annotations dictionary."""
        return self._clone()

    def unique_params(self, *optionaldict, **kwargs):
        """Return a copy with ``bindparam()`` elments replaced.

        Same functionality as ``params()``, except adds `unique=True`
        to affected bind parameters so that multiple statements can be
        used.

        """
        return self._params(True, optionaldict, kwargs)

    def params(self, *optionaldict, **kwargs):
        """Return a copy with ``bindparam()`` elments replaced.

        Returns a copy of this ClauseElement with ``bindparam()``
        elements replaced with values taken from the given dictionary::

          >>> clause = column('x') + bindparam('foo')
          >>> print clause.compile().params
          {'foo':None}
          >>> print clause.params({'foo':7}).compile().params
          {'foo':7}

        """
        return self._params(False, optionaldict, kwargs)

    def _params(self, unique, optionaldict, kwargs):
        if len(optionaldict) == 1:
            kwargs.update(optionaldict[0])
        elif len(optionaldict) > 1:
            raise exc.ArgumentError("params() takes zero or one positional dictionary argument")

        def visit_bindparam(bind):
            if bind.key in kwargs:
                bind.value = kwargs[bind.key]
            if unique:
                bind._convert_to_unique()
        return cloned_traverse(self, {}, {'bindparam':visit_bindparam})

    def compare(self, other):
        """Compare this ClauseElement to the given ClauseElement.

        Subclasses should override the default behavior, which is a
        straight identity comparison.

        """
        return self is other

    def _copy_internals(self, clone=_clone):
        """Reassign internal elements to be clones of themselves.

        Called during a copy-and-traverse operation on newly
        shallow-copied elements to create a deep copy.

        """
        pass

    def get_children(self, **kwargs):
        """Return immediate child elements of this ``ClauseElement``.

        This is used for visit traversal.

        \**kwargs may contain flags that change the collection that is
        returned, for example to return a subset of items in order to
        cut down on larger traversals, or to return child items from a
        different context (such as schema-level collections instead of
        clause-level).

        """
        return []

    def self_group(self, against=None):
        return self

    @property
    def bind(self):
        """Returns the Engine or Connection to which this ClauseElement is bound, or None if none found."""

        try:
            if self._bind is not None:
                return self._bind
        except AttributeError:
            pass
        for f in _from_objects(self):
            if f is self:
                continue
            engine = f.bind
            if engine is not None:
                return engine
        else:
            return None

    def execute(self, *multiparams, **params):
        """Compile and execute this ``ClauseElement``."""

        e = self.bind
        if e is None:
            label = getattr(self, 'description', self.__class__.__name__)
            msg = ('This %s is not bound and does not support direct '
                   'execution. Supply this statement to a Connection or '
                   'Engine for execution. Or, assign a bind to the statement '
                   'or the Metadata of its underlying tables to enable '
                   'implicit execution via this method.' % label)
            raise exc.UnboundExecutionError(msg)
        return e._execute_clauseelement(self, multiparams, params)

    def scalar(self, *multiparams, **params):
        """Compile and execute this ``ClauseElement``, returning the result's scalar representation."""

        return self.execute(*multiparams, **params).scalar()

    def compile(self, bind=None, column_keys=None, compiler=None, dialect=None, inline=False):
        """Compile this SQL expression.

        The return value is a :class:`~sqlalchemy.engine.Compiled` object.
        Calling `str()` or `unicode()` on the returned value will yield
        a string representation of the result.   The :class:`~sqlalchemy.engine.Compiled`
        object also can return a dictionary of bind parameter names and
        values using the `params` accessor.

        :param bind: An ``Engine`` or ``Connection`` from which a
          ``Compiled`` will be acquired.  This argument
          takes precedence over this ``ClauseElement``'s
          bound engine, if any.

        :param column_keys: Used for INSERT and UPDATE statements, a list of
          column names which should be present in the VALUES clause
          of the compiled statement.  If ``None``, all columns
          from the target table object are rendered.

        :param compiler: A ``Compiled`` instance which will be used to compile
          this expression.  This argument takes precedence
          over the `bind` and `dialect` arguments as well as
          this ``ClauseElement``'s bound engine, if
          any.

        :param dialect: A ``Dialect`` instance frmo which a ``Compiled``
          will be acquired.  This argument takes precedence
          over the `bind` argument as well as this
          ``ClauseElement``'s bound engine, if any.

        :param inline: Used for INSERT statements, for a dialect which does
          not support inline retrieval of newly generated
          primary key columns, will force the expression used
          to create the new primary key value to be rendered
          inline within the INSERT statement's VALUES clause.
          This typically refers to Sequence execution but
          may also refer to any server-side default generation
          function associated with a primary key `Column`.

        """
        if compiler is None:
            if dialect is not None:
                compiler = dialect.statement_compiler(dialect, self, column_keys=column_keys, inline=inline)
            elif bind is not None:
                compiler = bind.statement_compiler(self, column_keys=column_keys, inline=inline)
            elif self.bind is not None:
                compiler = self.bind.statement_compiler(self, column_keys=column_keys, inline=inline)
            else:
                global DefaultDialect
                if DefaultDialect is None:
                    from sqlalchemy.engine.default import DefaultDialect
                dialect = DefaultDialect()
                compiler = dialect.statement_compiler(dialect, self, column_keys=column_keys, inline=inline)
        compiler.compile()
        return compiler

    def __str__(self):
        return unicode(self.compile()).encode('ascii', 'backslashreplace')

    def __and__(self, other):
        return and_(self, other)

    def __or__(self, other):
        return or_(self, other)

    def __invert__(self):
        return self._negate()

    def _negate(self):
        if hasattr(self, 'negation_clause'):
            return self.negation_clause
        else:
            return _UnaryExpression(self.self_group(against=operators.inv), operator=operators.inv, negate=None)

    def __repr__(self):
        friendly = getattr(self, 'description', None)
        if friendly is None:
            return object.__repr__(self)
        else:
            return '<%s.%s at 0x%x; %s>' % (
                self.__module__, self.__class__.__name__, id(self), friendly)


class _Immutable(object):
    """mark a ClauseElement as 'immutable' when expressions are cloned."""

    def _clone(self):
        return self

class Operators(object):
    def __and__(self, other):
        return self.operate(operators.and_, other)

    def __or__(self, other):
        return self.operate(operators.or_, other)

    def __invert__(self):
        return self.operate(operators.inv)

    def op(self, opstring):
        def op(b):
            return self.operate(operators.op, opstring, b)
        return op

    def operate(self, op, *other, **kwargs):
        raise NotImplementedError(str(op))

    def reverse_operate(self, op, other, **kwargs):
        raise NotImplementedError(str(op))

class ColumnOperators(Operators):
    """Defines comparison and math operations."""

    timetuple = None
    """Hack, allows datetime objects to be compared on the LHS."""

    def __lt__(self, other):
        return self.operate(operators.lt, other)

    def __le__(self, other):
        return self.operate(operators.le, other)

    __hash__ = Operators.__hash__
    
    def __eq__(self, other):
        return self.operate(operators.eq, other)

    def __ne__(self, other):
        return self.operate(operators.ne, other)

    def __gt__(self, other):
        return self.operate(operators.gt, other)

    def __ge__(self, other):
        return self.operate(operators.ge, other)

    def concat(self, other):
        return self.operate(operators.concat_op, other)

    def like(self, other, escape=None):
        return self.operate(operators.like_op, other, escape=escape)

    def ilike(self, other, escape=None):
        return self.operate(operators.ilike_op, other, escape=escape)

    def in_(self, other):
        return self.operate(operators.in_op, other)

    def startswith(self, other, **kwargs):
        return self.operate(operators.startswith_op, other, **kwargs)

    def endswith(self, other, **kwargs):
        return self.operate(operators.endswith_op, other, **kwargs)

    def contains(self, other, **kwargs):
        return self.operate(operators.contains_op, other, **kwargs)

    def match(self, other, **kwargs):
        return self.operate(operators.match_op, other, **kwargs)

    def desc(self):
        return self.operate(operators.desc_op)

    def asc(self):
        return self.operate(operators.asc_op)

    def collate(self, collation):
        return self.operate(operators.collate, collation)

    def __radd__(self, other):
        return self.reverse_operate(operators.add, other)

    def __rsub__(self, other):
        return self.reverse_operate(operators.sub, other)

    def __rmul__(self, other):
        return self.reverse_operate(operators.mul, other)

    def __rdiv__(self, other):
        return self.reverse_operate(operators.div, other)

    def between(self, cleft, cright):
        return self.operate(operators.between_op, cleft, cright)

    def distinct(self):
        return self.operate(operators.distinct_op)

    def __add__(self, other):
        return self.operate(operators.add, other)

    def __sub__(self, other):
        return self.operate(operators.sub, other)

    def __mul__(self, other):
        return self.operate(operators.mul, other)

    def __div__(self, other):
        return self.operate(operators.div, other)

    def __mod__(self, other):
        return self.operate(operators.mod, other)

    def __truediv__(self, other):
        return self.operate(operators.truediv, other)

class _CompareMixin(ColumnOperators):
    """Defines comparison and math operations for ``ClauseElement`` instances."""

    def __compare(self, op, obj, negate=None, reverse=False, **kwargs):
        if obj is None or isinstance(obj, _Null):
            if op == operators.eq:
                return _BinaryExpression(self, null(), operators.is_, negate=operators.isnot)
            elif op == operators.ne:
                return _BinaryExpression(self, null(), operators.isnot, negate=operators.is_)
            else:
                raise exc.ArgumentError("Only '='/'!=' operators can be used with NULL")
        else:
            obj = self._check_literal(obj)

        if reverse:
            return _BinaryExpression(obj, self, op, type_=sqltypes.Boolean, negate=negate, modifiers=kwargs)
        else:
            return _BinaryExpression(self, obj, op, type_=sqltypes.Boolean, negate=negate, modifiers=kwargs)

    def __operate(self, op, obj, reverse=False):
        obj = self._check_literal(obj)

        type_ = self._compare_type(obj)

        if reverse:
            return _BinaryExpression(obj, self, type_.adapt_operator(op), type_=type_)
        else:
            return _BinaryExpression(self, obj, type_.adapt_operator(op), type_=type_)

    # a mapping of operators with the method they use, along with their negated
    # operator for comparison operators
    operators = {
        operators.add : (__operate,),
        operators.mul : (__operate,),
        operators.sub : (__operate,),
        operators.div : (__operate,),
        operators.mod : (__operate,),
        operators.truediv : (__operate,),
        operators.lt : (__compare, operators.ge),
        operators.le : (__compare, operators.gt),
        operators.ne : (__compare, operators.eq),
        operators.gt : (__compare, operators.le),
        operators.ge : (__compare, operators.lt),
        operators.eq : (__compare, operators.ne),
        operators.like_op : (__compare, operators.notlike_op),
        operators.ilike_op : (__compare, operators.notilike_op),
    }

    def operate(self, op, *other, **kwargs):
        o = _CompareMixin.operators[op]
        return o[0](self, op, other[0], *o[1:], **kwargs)

    def reverse_operate(self, op, other, **kwargs):
        o = _CompareMixin.operators[op]
        return o[0](self, op, other, reverse=True, *o[1:], **kwargs)

    def in_(self, other):
        return self._in_impl(operators.in_op, operators.notin_op, other)

    def _in_impl(self, op, negate_op, seq_or_selectable):
        seq_or_selectable = _clause_element_as_expr(seq_or_selectable)
            
        if isinstance(seq_or_selectable, _ScalarSelect):
            return self.__compare( op, seq_or_selectable, negate=negate_op)

        elif isinstance(seq_or_selectable, _SelectBaseMixin):
            # TODO: if we ever want to support (x, y, z) IN (select x, y, z from table),
            # we would need a multi-column version of as_scalar() to produce a multi-
            # column selectable that does not export itself as a FROM clause
            return self.__compare( op, seq_or_selectable.as_scalar(), negate=negate_op)

        elif isinstance(seq_or_selectable, Selectable):
            return self.__compare( op, seq_or_selectable, negate=negate_op)

        # Handle non selectable arguments as sequences
        args = []
        for o in seq_or_selectable:
            if not _is_literal(o):
                if not isinstance( o, _CompareMixin):
                    raise exc.InvalidRequestError( 
                        "in() function accepts either a list of non-selectable values, or a selectable: %r" % o)
            else:
                o = self._bind_param(o)
            args.append(o)

        if len(args) == 0:
            # Special case handling for empty IN's, behave like comparison against zero row selectable
            return self != self

        return self.__compare(op, ClauseList(*args).self_group(against=op), negate=negate_op)

    def startswith(self, other, escape=None):
        """Produce the clause ``LIKE '<other>%'``"""

        # use __radd__ to force string concat behavior
        return self.__compare(
            operators.like_op,
            literal_column("'%'", type_=sqltypes.String).__radd__(self._check_literal(other)),
            escape=escape)

    def endswith(self, other, escape=None):
        """Produce the clause ``LIKE '%<other>'``"""

        return self.__compare(
            operators.like_op,
            literal_column("'%'", type_=sqltypes.String) + self._check_literal(other),
            escape=escape)

    def contains(self, other, escape=None):
        """Produce the clause ``LIKE '%<other>%'``"""

        return self.__compare(
            operators.like_op,
            literal_column("'%'", type_=sqltypes.String) +
                self._check_literal(other) +
                literal_column("'%'", type_=sqltypes.String),
            escape=escape)

    def match(self, other):
        """Produce a MATCH clause, i.e. ``MATCH '<other>'``

        The allowed contents of ``other`` are database backend specific.

        """
        return self.__compare(operators.match_op, self._check_literal(other))

    def label(self, name):
        """Produce a column label, i.e. ``<columnname> AS <name>``.

        if 'name' is None, an anonymous label name will be generated.

        """
        return _Label(name, self, self.type)

    def desc(self):
        """Produce a DESC clause, i.e. ``<columnname> DESC``"""

        return desc(self)

    def asc(self):
        """Produce a ASC clause, i.e. ``<columnname> ASC``"""

        return asc(self)

    def distinct(self):
        """Produce a DISTINCT clause, i.e. ``DISTINCT <columnname>``"""
        return _UnaryExpression(self, operator=operators.distinct_op, type_=self.type)

    def between(self, cleft, cright):
        """Produce a BETWEEN clause, i.e. ``<column> BETWEEN <cleft> AND <cright>``"""

        return _BinaryExpression(
                self,
                ClauseList(
                    self._check_literal(cleft),
                    self._check_literal(cright),
                    operator=operators.and_,
                    group=False),
                operators.between_op)

    def collate(self, collation):
        """Produce a COLLATE clause, i.e. ``<column> COLLATE utf8_bin``"""

        return collate(self, collation)

    def op(self, operator):
        """produce a generic operator function.

        e.g.::

          somecolumn.op("*")(5)

        produces::

          somecolumn * 5

        operator
          a string which will be output as the infix operator between
          this ``ClauseElement`` and the expression passed to the
          generated function.

        """
        return lambda other: self.__operate(operator, other)

    def _bind_param(self, obj):
        return _BindParamClause(None, obj, type_=self.type, unique=True)

    def _check_literal(self, other):
        if isinstance(other, _BindParamClause) and isinstance(other.type, sqltypes.NullType):
            other.type = self.type
            return other
        elif hasattr(other, '__clause_element__'):
            return other.__clause_element__()
        elif not isinstance(other, ClauseElement):
            return self._bind_param(other)
        elif isinstance(other, (_SelectBaseMixin, Alias)):
            return other.as_scalar()
        else:
            return other

    def _compare_type(self, obj):
        """Allow subclasses to override the type used in constructing
        ``_BinaryExpression`` objects.

        Default return value is the type of the given object.

        """
        return obj.type

class ColumnElement(ClauseElement, _CompareMixin):
    """Represent an element that is usable within the "column clause" portion of a ``SELECT`` statement.

    This includes columns associated with tables, aliases, and
    subqueries, expressions, function calls, SQL keywords such as
    ``NULL``, literals, etc.  ``ColumnElement`` is the ultimate base
    class for all such elements.

    ``ColumnElement`` supports the ability to be a *proxy* element,
    which indicates that the ``ColumnElement`` may be associated with
    a ``Selectable`` which was derived from another ``Selectable``.
    An example of a "derived" ``Selectable`` is an ``Alias`` of a
    ``Table``.

    A ``ColumnElement``, by subclassing the ``_CompareMixin`` mixin
    class, provides the ability to generate new ``ClauseElement``
    objects using Python expressions.  See the ``_CompareMixin``
    docstring for more details.

    """

    __visit_name__ = 'column'
    primary_key = False
    foreign_keys = []
    quote = None
    _label = None
    
    @property
    def _select_iterable(self):
        return (self, )

    @util.memoized_property
    def base_columns(self):
        return util.column_set(c for c in self.proxy_set
                                     if not hasattr(c, 'proxies'))

    @util.memoized_property
    def proxy_set(self):
        s = util.column_set([self])
        if hasattr(self, 'proxies'):
            for c in self.proxies:
                s.update(c.proxy_set)
        return s

    def shares_lineage(self, othercolumn):
        """Return True if the given ``ColumnElement`` has a common ancestor to this ``ColumnElement``."""

        return bool(self.proxy_set.intersection(othercolumn.proxy_set))

    def _make_proxy(self, selectable, name=None):
        """Create a new ``ColumnElement`` representing this
        ``ColumnElement`` as it appears in the select list of a
        descending selectable.

        """

        if name:
            co = ColumnClause(name, selectable, type_=getattr(self, 'type', None))
        else:
            name = str(self)
            co = ColumnClause(self.anon_label, selectable, type_=getattr(self, 'type', None))

        co.proxies = [self]
        selectable.columns[name] = co
        return co

    @util.memoized_property
    def anon_label(self):
        """provides a constant 'anonymous label' for this ColumnElement.

        This is a label() expression which will be named at compile time.
        The same label() is returned each time anon_label is called so
        that expressions can reference anon_label multiple times, producing
        the same label name at compile time.

        the compiler uses this function automatically at compile time
        for expressions that are known to be 'unnamed' like binary
        expressions and function calls.

        """
        return _generated_label("%%(%d %s)s" % (id(self), getattr(self, 'name', 'anon')))

class ColumnCollection(util.OrderedProperties):
    """An ordered dictionary that stores a list of ColumnElement
    instances.

    Overrides the ``__eq__()`` method to produce SQL clauses between
    sets of correlated columns.

    """

    def __init__(self, *cols):
        super(ColumnCollection, self).__init__()
        [self.add(c) for c in cols]

    def __str__(self):
        return repr([str(c) for c in self])

    def replace(self, column):
        """add the given column to this collection, removing unaliased versions of this column
           as well as existing columns with the same key.

            e.g.::

                t = Table('sometable', metadata, Column('col1', Integer))
                t.columns.replace(Column('col1', Integer, key='columnone'))

            will remove the original 'col1' from the collection, and add
            the new column under the name 'columnname'.

           Used by schema.Column to override columns during table reflection.

        """
        if column.name in self and column.key != column.name:
            other = self[column.name]
            if other.name == other.key:
                del self[other.name]
        util.OrderedProperties.__setitem__(self, column.key, column)

    def add(self, column):
        """Add a column to this collection.

        The key attribute of the column will be used as the hash key
        for this dictionary.

        """
        self[column.key] = column

    def __setitem__(self, key, value):
        if key in self:
            # this warning is primarily to catch select() statements which have conflicting
            # column names in their exported columns collection
            existing = self[key]
            if not existing.shares_lineage(value):
                table = getattr(existing, 'table', None) and existing.table.description
                util.warn(("Column %r on table %r being replaced by another "
                           "column with the same key.  Consider use_labels "
                           "for select() statements.")  % (key, table))
        util.OrderedProperties.__setitem__(self, key, value)

    def remove(self, column):
        del self[column.key]

    def extend(self, iter):
        for c in iter:
            self.add(c)

    __hash__ = None
    
    def __eq__(self, other):
        l = []
        for c in other:
            for local in self:
                if c.shares_lineage(local):
                    l.append(c==local)
        return and_(*l)

    def __contains__(self, other):
        if not isinstance(other, basestring):
            raise exc.ArgumentError("__contains__ requires a string argument")
        return util.OrderedProperties.__contains__(self, other)

    def contains_column(self, col):
        # have to use a Set here, because it will compare the identity
        # of the column, not just using "==" for comparison which will always return a
        # "True" value (i.e. a BinaryClause...)
        return col in util.column_set(self)

class ColumnSet(util.ordered_column_set):
    def contains_column(self, col):
        return col in self

    def extend(self, cols):
        for col in cols:
            self.add(col)

    def __add__(self, other):
        return list(self) + list(other)

    def __eq__(self, other):
        l = []
        for c in other:
            for local in self:
                if c.shares_lineage(local):
                    l.append(c==local)
        return and_(*l)

    def __hash__(self):
        return hash(tuple(x for x in self))

class Selectable(ClauseElement):
    """mark a class as being selectable"""
    __visit_name__ = 'selectable'

class FromClause(Selectable):
    """Represent an element that can be used within the ``FROM`` clause of a ``SELECT`` statement."""

    __visit_name__ = 'fromclause'
    named_with_column = False
    _hide_froms = []
    quote = None
    schema = None

    def count(self, whereclause=None, **params):
        """return a SELECT COUNT generated against this ``FromClause``."""

        if self.primary_key:
            col = list(self.primary_key)[0]
        else:
            col = list(self.columns)[0]
        return select([func.count(col).label('tbl_row_count')], whereclause, from_obj=[self], **params)

    def select(self, whereclause=None, **params):
        """return a SELECT of this ``FromClause``."""

        return select([self], whereclause, **params)

    def join(self, right, onclause=None, isouter=False):
        """return a join of this ``FromClause`` against another ``FromClause``."""

        return Join(self, right, onclause, isouter)

    def outerjoin(self, right, onclause=None):
        """return an outer join of this ``FromClause`` against another ``FromClause``."""

        return Join(self, right, onclause, True)

    def alias(self, name=None):
        """return an alias of this ``FromClause``.
        
        For table objects, this has the effect of the table being rendered
        as ``tablename AS aliasname`` in a SELECT statement.  
        For select objects, the effect is that of creating a named
        subquery, i.e. ``(select ...) AS aliasname``.
        The ``alias()`` method is the general way to create
        a "subquery" out of an existing SELECT.
        
        The ``name`` parameter is optional, and if left blank an 
        "anonymous" name will be generated at compile time, guaranteed
        to be unique against other anonymous constructs used in the
        same statement.
        
        """

        return Alias(self, name)

    def is_derived_from(self, fromclause):
        """Return True if this FromClause is 'derived' from the given FromClause.

        An example would be an Alias of a Table is derived from that Table.

        """
        return fromclause in self._cloned_set

    def replace_selectable(self, old, alias):
        """replace all occurences of FromClause 'old' with the given Alias object, returning a copy of this ``FromClause``."""

        global ClauseAdapter
        if ClauseAdapter is None:
            from sqlalchemy.sql.util import ClauseAdapter
        return ClauseAdapter(alias).traverse(self)

    def correspond_on_equivalents(self, column, equivalents):
        """Return corresponding_column for the given column, or if None
        search for a match in the given dictionary.
        
        """
        col = self.corresponding_column(column, require_embedded=True)
        if col is None and col in equivalents:
            for equiv in equivalents[col]:
                nc = self.corresponding_column(equiv, require_embedded=True)
                if nc:
                    return nc
        return col

    def corresponding_column(self, column, require_embedded=False):
        """Given a ``ColumnElement``, return the exported ``ColumnElement``
        object from this ``Selectable`` which corresponds to that
        original ``Column`` via a common anscestor column.

        :param column: the target ``ColumnElement`` to be matched

        :param require_embedded: only return corresponding columns for the given
          ``ColumnElement``, if the given ``ColumnElement`` is
          actually present within a sub-element of this
          ``FromClause``.  Normally the column will match if it merely
          shares a common anscestor with one of the exported columns
          of this ``FromClause``.

        """
        # dont dig around if the column is locally present
        if self.c.contains_column(column):
            return column

        col, intersect = None, None
        target_set = column.proxy_set
        cols = self.c
        for c in cols:
            i = c.proxy_set.intersection(target_set)
            if i and \
                (not require_embedded or c.proxy_set.issuperset(target_set)):
                
                if col is None:
                    # no corresponding column yet, pick this one.
                    col, intersect = c, i
                elif len(i) > len(intersect):
                    # 'c' has a larger field of correspondence than 'col'.
                    # i.e. selectable.c.a1_x->a1.c.x->table.c.x matches a1.c.x->table.c.x better than 
                    # selectable.c.x->table.c.x does.
                    col, intersect = c, i
                elif i == intersect:
                    # they have the same field of correspondence.
                    # see which proxy_set has fewer columns in it, which indicates a
                    # closer relationship with the root column.  Also take into account the 
                    # "weight" attribute which CompoundSelect() uses to give higher precedence to
                    # columns based on vertical position in the compound statement, and discard columns
                    # that have no reference to the target column (also occurs with CompoundSelect)
                    col_distance = util.reduce(operator.add, 
                                        [sc._annotations.get('weight', 1) for sc in col.proxy_set if sc.shares_lineage(column)]
                                    )
                    c_distance = util.reduce(operator.add, 
                                        [sc._annotations.get('weight', 1) for sc in c.proxy_set if sc.shares_lineage(column)]
                                    )
                    if \
                        c_distance < col_distance:
                        col, intersect = c, i
        return col

    @property
    def description(self):
        """a brief description of this FromClause.

        Used primarily for error message formatting.

        """
        return getattr(self, 'name', self.__class__.__name__ + " object")

    def _reset_exported(self):
        """delete memoized collections when a FromClause is cloned."""

        for attr in ('_columns', '_primary_key' '_foreign_keys', 'locate_all_froms'):
            self.__dict__.pop(attr, None)

    @util.memoized_property
    def _columns(self):
        """Return the collection of Column objects contained by this FromClause."""

        self._export_columns()
        return self._columns

    @util.memoized_property
    def _primary_key(self):
        """Return the collection of Column objects which comprise the primary key of this FromClause."""

        self._export_columns()
        return self._primary_key

    @util.memoized_property
    def _foreign_keys(self):
        """Return the collection of ForeignKey objects which this FromClause references."""

        self._export_columns()
        return self._foreign_keys

    columns = property(attrgetter('_columns'), doc=_columns.__doc__)
    primary_key = property(attrgetter('_primary_key'), doc=_primary_key.__doc__)
    foreign_keys = property(attrgetter('_foreign_keys'), doc=_foreign_keys.__doc__)

    # synonyms for 'columns'
    c = _select_iterable = property(attrgetter('columns'), doc=_columns.__doc__)

    def _export_columns(self):
        """Initialize column collections."""

        self._columns = ColumnCollection()
        self._primary_key = ColumnSet()
        self._foreign_keys = set()
        self._populate_column_collection()

    def _populate_column_collection(self):
        pass

class _BindParamClause(ColumnElement):
    """Represent a bind parameter.

    Public constructor is the ``bindparam()`` function.

    """

    __visit_name__ = 'bindparam'
    quote = None

    def __init__(self, key, value, type_=None, unique=False, isoutparam=False, shortname=None):
        """Construct a _BindParamClause.

        key
          the key for this bind param.  Will be used in the generated
          SQL statement for dialects that use named parameters.  This
          value may be modified when part of a compilation operation,
          if other ``_BindParamClause`` objects exist with the same
          key, or if its length is too long and truncation is
          required.

        value
          Initial value for this bind param.  This value may be
          overridden by the dictionary of parameters sent to statement
          compilation/execution.

        shortname
          deprecated.

        type\_
          A ``TypeEngine`` object that will be used to pre-process the
          value corresponding to this ``_BindParamClause`` at
          execution time.

        unique
          if True, the key name of this BindParamClause will be
          modified if another ``_BindParamClause`` of the same name
          already has been located within the containing
          ``ClauseElement``.

        isoutparam
          if True, the parameter should be treated like a stored procedure "OUT"
          parameter.

        """
        if unique:
            self.key = _generated_label("%%(%d %s)s" % (id(self), key or 'param'))
        else:
            self.key = key or _generated_label("%%(%d param)s" % id(self))
        self._orig_key = key or 'param'
        self.unique = unique
        self.value = value
        self.isoutparam = isoutparam
        self.shortname = shortname

        if type_ is None:
            self.type = sqltypes.type_map.get(type(value), sqltypes.NullType)()
        elif isinstance(type_, type):
            self.type = type_()
        else:
            self.type = type_

    def _clone(self):
        c = ClauseElement._clone(self)
        if self.unique:
            c.key = _generated_label("%%(%d %s)s" % (id(c), c._orig_key or 'param'))
        return c

    def _convert_to_unique(self):
        if not self.unique:
            self.unique = True
            self.key = _generated_label("%%(%d %s)s" % (id(self), self._orig_key or 'param'))

    def bind_processor(self, dialect):
        return self.type.dialect_impl(dialect).bind_processor(dialect)

    def _compare_type(self, obj):
        if not isinstance(self.type, sqltypes.NullType):
            return self.type
        else:
            return obj.type

    def compare(self, other):
        """Compare this ``_BindParamClause`` to the given clause.

        Since ``compare()`` is meant to compare statement syntax, this
        method returns True if the two ``_BindParamClauses`` have just
        the same type.

        """
        return isinstance(other, _BindParamClause) and other.type.__class__ == self.type.__class__ and self.value == other.value

    def __getstate__(self):
        """execute a deferred value for serialization purposes."""

        d = self.__dict__.copy()
        v = self.value
        if util.callable(v):
            v = v()
        d['value'] = v
        return d

    def __repr__(self):
        return "_BindParamClause(%s, %s, type_=%s)" % (repr(self.key), repr(self.value), repr(self.type))

class _TypeClause(ClauseElement):
    """Handle a type keyword in a SQL statement.

    Used by the ``Case`` statement.

    """

    __visit_name__ = 'typeclause'

    def __init__(self, type):
        self.type = type


class _TextClause(ClauseElement):
    """Represent a literal SQL text fragment.

    Public constructor is the ``text()`` function.

    """

    __visit_name__ = 'textclause'

    _bind_params_regex = re.compile(r'(?<![:\w\x5c]):(\w+)(?!:)', re.UNICODE)
    supports_execution = True

    @property
    def _select_iterable(self):
        return (self,)

    _hide_froms = []

    def __init__(self, text = "", bind=None, bindparams=None, typemap=None, autocommit=False):
        self._bind = bind
        self.bindparams = {}
        self.typemap = typemap
        self._autocommit = autocommit
        if typemap is not None:
            for key in typemap.keys():
                typemap[key] = sqltypes.to_instance(typemap[key])

        def repl(m):
            self.bindparams[m.group(1)] = bindparam(m.group(1))
            return ":%s" % m.group(1)

        # scan the string and search for bind parameter names, add them
        # to the list of bindparams
        self.text = self._bind_params_regex.sub(repl, text)
        if bindparams is not None:
            for b in bindparams:
                self.bindparams[b.key] = b
                
    @property
    def type(self):
        if self.typemap is not None and len(self.typemap) == 1:
            return list(self.typemap)[0]
        else:
            return None

    def _copy_internals(self, clone=_clone):
        self.bindparams = dict((b.key, clone(b))
                               for b in self.bindparams.values())

    def get_children(self, **kwargs):
        return self.bindparams.values()


class _Null(ColumnElement):
    """Represent the NULL keyword in a SQL statement.

    Public constructor is the ``null()`` function.

    """

    __visit_name__ = 'null'

    def __init__(self):
        self.type = sqltypes.NULLTYPE


class ClauseList(ClauseElement):
    """Describe a list of clauses, separated by an operator.

    By default, is comma-separated, such as a column listing.

    """
    __visit_name__ = 'clauselist'

    def __init__(self, *clauses, **kwargs):
        self.operator = kwargs.pop('operator', operators.comma_op)
        self.group = kwargs.pop('group', True)
        self.group_contents = kwargs.pop('group_contents', True)
        if self.group_contents:
            self.clauses = [
                _literal_as_text(clause).self_group(against=self.operator)
                for clause in clauses if clause is not None]
        else:
            self.clauses = [
                _literal_as_text(clause)
                for clause in clauses if clause is not None]

    def __iter__(self):
        return iter(self.clauses)

    def __len__(self):
        return len(self.clauses)

    @property
    def _select_iterable(self):
        return iter(self)

    def append(self, clause):
        # TODO: not sure if i like the 'group_contents' flag.  need to
        # define the difference between a ClauseList of ClauseLists,
        # and a "flattened" ClauseList of ClauseLists.  flatten()
        # method ?
        if self.group_contents:
            self.clauses.append(_literal_as_text(clause).self_group(against=self.operator))
        else:
            self.clauses.append(_literal_as_text(clause))

    def _copy_internals(self, clone=_clone):
        self.clauses = [clone(clause) for clause in self.clauses]

    def get_children(self, **kwargs):
        return self.clauses

    @property
    def _from_objects(self):
        return list(itertools.chain(*[c._from_objects for c in self.clauses]))

    def self_group(self, against=None):
        if self.group and self.operator is not against and operators.is_precedent(self.operator, against):
            return _Grouping(self)
        else:
            return self

    def compare(self, other):
        """Compare this ``ClauseList`` to the given ``ClauseList``,
        including a comparison of all the clause items.

        """
        if not isinstance(other, ClauseList) and len(self.clauses) == 1:
            return self.clauses[0].compare(other)
        elif isinstance(other, ClauseList) and len(self.clauses) == len(other.clauses):
            for i in range(0, len(self.clauses)):
                if not self.clauses[i].compare(other.clauses[i]):
                    return False
            else:
                return self.operator == other.operator
        else:
            return False

class BooleanClauseList(ClauseList, ColumnElement):
    __visit_name__ = 'clauselist'

    def __init__(self, *clauses, **kwargs):
        super(BooleanClauseList, self).__init__(*clauses, **kwargs)
        self.type = sqltypes.to_instance(kwargs.get('type_', sqltypes.Boolean))

    @property
    def _select_iterable(self):
        return (self, )


class _Case(ColumnElement):
    __visit_name__ = 'case'

    def __init__(self, whens, value=None, else_=None):
        try:
            whens = util.dictlike_iteritems(whens)
        except TypeError:
            pass

        if value:
            whenlist = [(_literal_as_binds(c).self_group(), _literal_as_binds(r)) for (c, r) in whens]
        else:
            whenlist = [(_no_literals(c).self_group(), _literal_as_binds(r)) for (c, r) in whens]

        if whenlist:
            type_ = list(whenlist[-1])[-1].type
        else:
            type_ = None

        if value is None:
            self.value = None
        else:
            self.value = _literal_as_binds(value)

        self.type = type_
        self.whens = whenlist
        if else_ is not None:
            self.else_ = _literal_as_binds(else_)
        else:
            self.else_ = None

    def _copy_internals(self, clone=_clone):
        if self.value:
            self.value = clone(self.value)
        self.whens = [(clone(x), clone(y)) for x, y in self.whens]
        if self.else_:
            self.else_ = clone(self.else_)

    def get_children(self, **kwargs):
        if self.value:
            yield self.value
        for x, y in self.whens:
            yield x
            yield y
        if self.else_:
            yield self.else_ 

    @property
    def _from_objects(self):
        return list(itertools.chain(*[x._from_objects for x in self.get_children()]))

class Function(ColumnElement, FromClause):
    """Describe a SQL function."""

    __visit_name__ = 'function'

    def __init__(self, name, *clauses, **kwargs):
        self.packagenames = kwargs.get('packagenames', None) or []
        self.name = name
        self._bind = kwargs.get('bind', None)
        args = [_literal_as_binds(c, self.name) for c in clauses]
        self.clause_expr = ClauseList(operator=operators.comma_op, group_contents=True, *args).self_group()
        self.type = sqltypes.to_instance(kwargs.get('type_', None))

    @property
    def key(self):
        return self.name

    @property
    def columns(self):
        return [self]

    @util.memoized_property
    def clauses(self):
        return self.clause_expr.element
        
    @property
    def _from_objects(self):
        return self.clauses._from_objects

    def get_children(self, **kwargs):
        return self.clause_expr, 

    def _copy_internals(self, clone=_clone):
        self.clause_expr = clone(self.clause_expr)
        self._reset_exported()
        util.reset_memoized(self, 'clauses')
        
    def _bind_param(self, obj):
        return _BindParamClause(self.name, obj, type_=self.type, unique=True)

    def select(self):
        return select([self])

    def scalar(self):
        return select([self]).execute().scalar()

    def execute(self):
        return select([self]).execute()

    def _compare_type(self, obj):
        return self.type


class _Cast(ColumnElement):

    __visit_name__ = 'cast'

    def __init__(self, clause, totype, **kwargs):
        self.type = sqltypes.to_instance(totype)
        self.clause = _literal_as_binds(clause, None)
        self.typeclause = _TypeClause(self.type)

    def _copy_internals(self, clone=_clone):
        self.clause = clone(self.clause)
        self.typeclause = clone(self.typeclause)

    def get_children(self, **kwargs):
        return self.clause, self.typeclause

    @property
    def _from_objects(self):
        return self.clause._from_objects


class _Extract(ColumnElement):

    __visit_name__ = 'extract'

    def __init__(self, field, expr, **kwargs):
        self.type = sqltypes.Integer()
        self.field = field
        self.expr = _literal_as_binds(expr, None)

    def _copy_internals(self, clone=_clone):
        self.expr = clone(self.expr)

    def get_children(self, **kwargs):
        return self.expr,

    @property
    def _from_objects(self):
        return self.expr._from_objects


class _UnaryExpression(ColumnElement):

    __visit_name__ = 'unary'

    def __init__(self, element, operator=None, modifier=None, type_=None, negate=None):
        self.operator = operator
        self.modifier = modifier

        self.element = _literal_as_text(element).self_group(against=self.operator or self.modifier)
        self.type = sqltypes.to_instance(type_)
        self.negate = negate

    @property
    def _from_objects(self):
        return self.element._from_objects

    def _copy_internals(self, clone=_clone):
        self.element = clone(self.element)

    def get_children(self, **kwargs):
        return self.element,

    def compare(self, other):
        """Compare this ``_UnaryExpression`` against the given ``ClauseElement``."""

        return (
            isinstance(other, _UnaryExpression) and
            self.operator == other.operator and
            self.modifier == other.modifier and
            self.element.compare(other.element)
        )

    def _negate(self):
        if self.negate is not None:
            return _UnaryExpression(
                self.element,
                operator=self.negate,
                negate=self.operator,
                modifier=self.modifier,
                type_=self.type)
        else:
            return super(_UnaryExpression, self)._negate()

    def self_group(self, against):
        if self.operator and operators.is_precedent(self.operator, against):
            return _Grouping(self)
        else:
            return self


class _BinaryExpression(ColumnElement):
    """Represent an expression that is ``LEFT <operator> RIGHT``."""

    __visit_name__ = 'binary'

    def __init__(self, left, right, operator, type_=None, negate=None, modifiers=None):
        self.left = _literal_as_text(left).self_group(against=operator)
        self.right = _literal_as_text(right).self_group(against=operator)
        self.operator = operator
        self.type = sqltypes.to_instance(type_)
        self.negate = negate
        if modifiers is None:
            self.modifiers = {}
        else:
            self.modifiers = modifiers

    @property
    def _from_objects(self):
        return self.left._from_objects + self.right._from_objects

    def _copy_internals(self, clone=_clone):
        self.left = clone(self.left)
        self.right = clone(self.right)

    def get_children(self, **kwargs):
        return self.left, self.right

    def compare(self, other):
        """Compare this ``_BinaryExpression`` against the given ``_BinaryExpression``."""

        return (
            isinstance(other, _BinaryExpression) and
            self.operator == other.operator and
            (
                self.left.compare(other.left) and
                self.right.compare(other.right) or
                (
                    operators.is_commutative(self.operator) and
                    self.left.compare(other.right) and
                    self.right.compare(other.left)
                )
            )
        )

    def self_group(self, against=None):
        # use small/large defaults for comparison so that unknown
        # operators are always parenthesized
        if self.operator is not against and operators.is_precedent(self.operator, against):
            return _Grouping(self)
        else:
            return self

    def _negate(self):
        if self.negate is not None:
            return _BinaryExpression(
                self.left,
                self.right,
                self.negate,
                negate=self.operator,
                type_=self.type,
                modifiers=self.modifiers)
        else:
            return super(_BinaryExpression, self)._negate()

class _Exists(_UnaryExpression):
    __visit_name__ = _UnaryExpression.__visit_name__
    _from_objects = []

    def __init__(self, *args, **kwargs):
        if args and isinstance(args[0], _SelectBaseMixin):
            s = args[0]
        else:
            if not args:
                args = ([literal_column('*')],)
            s = select(*args, **kwargs).as_scalar().self_group()

        _UnaryExpression.__init__(self, s, operator=operators.exists, type_=sqltypes.Boolean)

    def select(self, whereclause=None, **params):
        return select([self], whereclause, **params)

    def correlate(self, fromclause):
        e = self._clone()
        e.element = self.element.correlate(fromclause).self_group()
        return e

    def select_from(self, clause):
        """return a new exists() construct with the given expression set as its FROM clause."""

        e = self._clone()
        e.element = self.element.select_from(clause).self_group()
        return e

    def where(self, clause):
        """return a new exists() construct with the given expression added to its WHERE clause, joined
        to the existing clause via AND, if any."""

        e = self._clone()
        e.element = self.element.where(clause).self_group()
        return e

class Join(FromClause):
    """represent a ``JOIN`` construct between two ``FromClause`` elements.

    The public constructor function for ``Join`` is the module-level
    ``join()`` function, as well as the ``join()`` method available
    off all ``FromClause`` subclasses.

    """
    __visit_name__ = 'join'

    def __init__(self, left, right, onclause=None, isouter=False):
        self.left = _literal_as_text(left)
        self.right = _literal_as_text(right).self_group()

        if onclause is None:
            self.onclause = self._match_primaries(self.left, self.right)
        else:
            self.onclause = onclause

        self.isouter = isouter
        self.__folded_equivalents = None

    @property
    def description(self):
        return "Join object on %s(%d) and %s(%d)" % (
            self.left.description,
            id(self.left),
            self.right.description,
            id(self.right))

    def is_derived_from(self, fromclause):
        return fromclause is self or self.left.is_derived_from(fromclause) or self.right.is_derived_from(fromclause)

    def self_group(self, against=None):
        return _FromGrouping(self)

    def _populate_column_collection(self):
        columns = [c for c in self.left.columns] + [c for c in self.right.columns]

        global sql_util
        if not sql_util:
            from sqlalchemy.sql import util as sql_util
        self._primary_key.extend(sql_util.reduce_columns(
                (c for c in columns if c.primary_key), self.onclause))
        self._columns.update((col._label, col) for col in columns)
        self._foreign_keys.update(itertools.chain(*[col.foreign_keys for col in columns]))

    def _copy_internals(self, clone=_clone):
        self._reset_exported()
        self.left = clone(self.left)
        self.right = clone(self.right)
        self.onclause = clone(self.onclause)
        self.__folded_equivalents = None

    def get_children(self, **kwargs):
        return self.left, self.right, self.onclause

    def _match_primaries(self, primary, secondary):
        global sql_util
        if not sql_util:
            from sqlalchemy.sql import util as sql_util
        return sql_util.join_condition(primary, secondary)

    def select(self, whereclause=None, fold_equivalents=False, **kwargs):
        """Create a :class:`Select` from this :class:`Join`.

        :param whereclause: the WHERE criterion that will be sent to 
          the :func:`select()` function

        :param fold_equivalents: based on the join criterion of this 
          :class:`Join`, do not include
          repeat column names in the column list of the resulting
          select, for columns that are calculated to be "equivalent"
          based on the join criterion of this :class:`Join`. This will
          recursively apply to any joins directly nested by this one
          as well.  This flag is specific to a particular use case
          by the ORM and will be deprecated in 0.6.

        :param \**kwargs: all other kwargs are sent to the 
          underlying :func:`select()` function.

        """
        if fold_equivalents:
            global sql_util
            if not sql_util:
                from sqlalchemy.sql import util as sql_util
            collist = sql_util.folded_equivalents(self)
        else:
            collist = [self.left, self.right]

        return select(collist, whereclause, from_obj=[self], **kwargs)

    @property
    def bind(self):
        return self.left.bind or self.right.bind

    def alias(self, name=None):
        """Create a ``Select`` out of this ``Join`` clause and return an ``Alias`` of it.

        The ``Select`` is not correlating.

        """
        return self.select(use_labels=True, correlate=False).alias(name)

    @property
    def _hide_froms(self):
        return itertools.chain(*[_from_objects(x.left, x.right) for x in self._cloned_set])

    @property
    def _from_objects(self):
        return [self] + \
                self.onclause._from_objects + \
                self.left._from_objects + \
                self.right._from_objects

class Alias(FromClause):
    """Represents an table or selectable alias (AS).

    Represents an alias, as typically applied to any table or
    sub-select within a SQL statement using the ``AS`` keyword (or
    without the keyword on certain databases such as Oracle).

    This object is constructed from the ``alias()`` module level
    function as well as the ``alias()`` method available on all
    ``FromClause`` subclasses.

    """

    __visit_name__ = 'alias'
    named_with_column = True

    def __init__(self, selectable, alias=None):
        baseselectable = selectable
        while isinstance(baseselectable, Alias):
            baseselectable = baseselectable.element
        self.original = baseselectable
        self.supports_execution = baseselectable.supports_execution
        if self.supports_execution:
            self._autocommit = baseselectable._autocommit
        self.element = selectable
        if alias is None:
            if self.original.named_with_column:
                alias = getattr(self.original, 'name', None)
            alias = _generated_label('%%(%d %s)s' % (id(self), alias or 'anon'))
        self.name = alias

    @property
    def description(self):
        return self.name.encode('ascii', 'backslashreplace')

    def as_scalar(self):
        try:
            return self.element.as_scalar()
        except AttributeError:
            raise AttributeError("Element %s does not support 'as_scalar()'" % self.element)
        
    def is_derived_from(self, fromclause):
        if fromclause in self._cloned_set:
            return True
        return self.element.is_derived_from(fromclause)

    def _populate_column_collection(self):
        for col in self.element.columns:
            col._make_proxy(self)

    def _copy_internals(self, clone=_clone):
        self._reset_exported()
        self.element = _clone(self.element)
        baseselectable = self.element
        while isinstance(baseselectable, Alias):
            baseselectable = baseselectable.selectable
        self.original = baseselectable

    def get_children(self, column_collections=True, aliased_selectables=True, **kwargs):
        if column_collections:
            for c in self.c:
                yield c
        if aliased_selectables:
            yield self.element

    @property
    def _from_objects(self):
        return [self]

    @property
    def bind(self):
        return self.element.bind

class _Grouping(ColumnElement):
    """Represent a grouping within a column expression"""

    __visit_name__ = 'grouping'

    def __init__(self, element):
        self.element = element
        self.type = getattr(element, 'type', None)

    @property
    def key(self):
        return self.element.key

    @property
    def _label(self):
        return getattr(self.element, '_label', None) or self.anon_label

    def _copy_internals(self, clone=_clone):
        self.element = clone(self.element)

    def get_children(self, **kwargs):
        return self.element,

    @property
    def _from_objects(self):
        return self.element._from_objects

    def __getattr__(self, attr):
        return getattr(self.element, attr)

    def __getstate__(self):
        return {'element':self.element, 'type':self.type}

    def __setstate__(self, state):
        self.element = state['element']
        self.type = state['type']

class _FromGrouping(FromClause):
    """Represent a grouping of a FROM clause"""
    __visit_name__ = 'grouping'

    def __init__(self, element):
        self.element = element

    @property
    def columns(self):
        return self.element.columns

    @property
    def _hide_froms(self):
        return self.element._hide_froms

    def get_children(self, **kwargs):
        return self.element,

    def _copy_internals(self, clone=_clone):
        self.element = clone(self.element)

    @property
    def _from_objects(self):
        return self.element._from_objects

    def __getattr__(self, attr):
        return getattr(self.element, attr)

    def __getstate__(self):
        return {'element':self.element}

    def __setstate__(self, state):
        self.element = state['element']

class _Label(ColumnElement):
    """Represents a column label (AS).

    Represent a label, as typically applied to any column-level
    element using the ``AS`` sql keyword.

    This object is constructed from the ``label()`` module level
    function as well as the ``label()`` method available on all
    ``ColumnElement`` subclasses.

    """

    __visit_name__ = 'label'

    def __init__(self, name, element, type_=None):
        while isinstance(element, _Label):
            element = element.element
        self.name = self.key = self._label = name or _generated_label("%%(%d %s)s" % (id(self), getattr(element, 'name', 'anon')))
        self._element = element
        self._type = type_
        self.quote = element.quote

    @util.memoized_property
    def type(self):
        return sqltypes.to_instance(self._type or getattr(self._element, 'type', None))

    @util.memoized_property
    def element(self):
        return self._element.self_group(against=operators.as_)

    def _proxy_attr(name):
        get = attrgetter(name)
        def attr(self):
            return get(self.element)
        return property(attr)

    proxies = _proxy_attr('proxies')
    base_columns = _proxy_attr('base_columns')
    proxy_set = _proxy_attr('proxy_set')
    primary_key = _proxy_attr('primary_key')
    foreign_keys = _proxy_attr('foreign_keys')

    def get_children(self, **kwargs):
        return self.element,

    def _copy_internals(self, clone=_clone):
        self.element = clone(self.element)

    @property
    def _from_objects(self):
        return self.element._from_objects

    def _make_proxy(self, selectable, name = None):
        if isinstance(self.element, (Selectable, ColumnElement)):
            e = self.element._make_proxy(selectable, name=self.name)
        else:
            e = column(self.name)._make_proxy(selectable=selectable)
        e.proxies.append(self)
        return e

class ColumnClause(_Immutable, ColumnElement):
    """Represents a generic column expression from any textual string.

    This includes columns associated with tables, aliases and select
    statements, but also any arbitrary text.  May or may not be bound
    to an underlying ``Selectable``.  ``ColumnClause`` is usually
    created publically via the ``column()`` function or the
    ``literal_column()`` function.

    text
      the text of the element.

    selectable
      parent selectable.

    type
      ``TypeEngine`` object which can associate this ``ColumnClause``
      with a type.

    is_literal
      if True, the ``ColumnClause`` is assumed to be an exact
      expression that will be delivered to the output with no quoting
      rules applied regardless of case sensitive settings.  the
      ``literal_column()`` function is usually used to create such a
      ``ColumnClause``.

    """
    __visit_name__ = 'column'

    def __init__(self, text, selectable=None, type_=None, is_literal=False):
        self.key = self.name = text
        self.table = selectable
        self.type = sqltypes.to_instance(type_)
        self.is_literal = is_literal

    @util.memoized_property
    def description(self):
        return self.name.encode('ascii', 'backslashreplace')

    @util.memoized_property
    def _label(self):
        if self.is_literal:
            return None

        elif self.table and self.table.named_with_column:
            if getattr(self.table, 'schema', None):
                label = self.table.schema + "_" + \
                            _escape_for_generated(self.table.name) + "_" + \
                            _escape_for_generated(self.name)
            else:
                label = _escape_for_generated(self.table.name) + "_" + \
                            _escape_for_generated(self.name)

            if label in self.table.c:
                # TODO: coverage does not seem to be present for this
                _label = label
                counter = 1
                while _label in self.table.c:
                    _label = label + "_" + str(counter)
                    counter += 1
                label = _label
            return _generated_label(label)

        else:
            return self.name

    def label(self, name):
        if name is None:
            return self
        else:
            return super(ColumnClause, self).label(name)

    @property
    def _from_objects(self):
        if self.table:
            return [self.table]
        else:
            return []

    def _bind_param(self, obj):
        return _BindParamClause(self.name, obj, type_=self.type, unique=True)

    def _make_proxy(self, selectable, name=None, attach=True):
        # propagate the "is_literal" flag only if we are keeping our name,
        # otherwise its considered to be a label
        is_literal = self.is_literal and (name is None or name == self.name)
        c = ColumnClause(name or self.name, selectable=selectable, type_=self.type, is_literal=is_literal)
        c.proxies = [self]
        if attach:
            selectable.columns[c.name] = c
        return c

    def _compare_type(self, obj):
        return self.type

class TableClause(_Immutable, FromClause):
    """Represents a "table" construct.

    Note that this represents tables only as another syntactical
    construct within SQL expressions; it does not provide schema-level
    functionality.

    """

    __visit_name__ = 'table'

    named_with_column = True

    def __init__(self, name, *columns):
        super(TableClause, self).__init__()
        self.name = self.fullname = name
        self._columns = ColumnCollection()
        self._primary_key = ColumnSet()
        self._foreign_keys = set()
        for c in columns:
            self.append_column(c)

    def _export_columns(self):
        raise NotImplementedError()

    @util.memoized_property
    def description(self):
        return self.name.encode('ascii', 'backslashreplace')

    def append_column(self, c):
        self._columns[c.name] = c
        c.table = self

    def get_children(self, column_collections=True, **kwargs):
        if column_collections:
            return [c for c in self.c]
        else:
            return []

    def count(self, whereclause=None, **params):
        if self.primary_key:
            col = list(self.primary_key)[0]
        else:
            col = list(self.columns)[0]
        return select([func.count(col).label('tbl_row_count')], whereclause, from_obj=[self], **params)

    def insert(self, values=None, inline=False, **kwargs):
        """Generate an :func:`~sqlalchemy.sql.expression.insert()` construct."""

        return insert(self, values=values, inline=inline, **kwargs)

    def update(self, whereclause=None, values=None, inline=False, **kwargs):
        """Generate an :func:`~sqlalchemy.sql.expression.update()` construct."""

        return update(self, whereclause=whereclause, values=values, inline=inline, **kwargs)

    def delete(self, whereclause=None, **kwargs):
        """Generate a :func:`~sqlalchemy.sql.expression.delete()` construct."""

        return delete(self, whereclause, **kwargs)

    @property
    def _from_objects(self):
        return [self]

@util.decorator
def _generative(fn, *args, **kw):
    """Mark a method as generative."""

    self = args[0]._generate()
    fn(self, *args[1:], **kw)
    return self

class _SelectBaseMixin(object):
    """Base class for ``Select`` and ``CompoundSelects``."""

    supports_execution = True

    def __init__(self,
            use_labels=False,
            for_update=False,
            limit=None,
            offset=None,
            order_by=None,
            group_by=None,
            bind=None,
            autocommit=False):
        self.use_labels = use_labels
        self.for_update = for_update
        self._autocommit = autocommit
        self._limit = limit
        self._offset = offset
        self._bind = bind

        self._order_by_clause = ClauseList(*util.to_list(order_by) or [])
        self._group_by_clause = ClauseList(*util.to_list(group_by) or [])

    def as_scalar(self):
        """return a 'scalar' representation of this selectable, which can be used
        as a column expression.

        Typically, a select statement which has only one column in its columns clause
        is eligible to be used as a scalar expression.

        The returned object is an instance of :class:`~sqlalchemy.sql.expression._ScalarSelect`.

        """
        return _ScalarSelect(self)

    @_generative
    def apply_labels(self):
        """return a new selectable with the 'use_labels' flag set to True.

        This will result in column expressions being generated using labels against their table
        name, such as "SELECT somecolumn AS tablename_somecolumn".  This allows selectables which
        contain multiple FROM clauses to produce a unique set of column names regardless of name conflicts
        among the individual FROM clauses.

        """
        self.use_labels = True

    def label(self, name):
        """return a 'scalar' representation of this selectable, embedded as a subquery
        with a label.

        See also ``as_scalar()``.

        """
        return self.as_scalar().label(name)

    @_generative
    def autocommit(self):
        """return a new selectable with the 'autocommit' flag set to True."""

        self._autocommit = True

    def _generate(self):
        s = self.__class__.__new__(self.__class__)
        s.__dict__ = self.__dict__.copy()
        s._reset_exported()
        return s

    @_generative
    def limit(self, limit):
        """return a new selectable with the given LIMIT criterion applied."""

        self._limit = limit

    @_generative
    def offset(self, offset):
        """return a new selectable with the given OFFSET criterion applied."""

        self._offset = offset

    @_generative
    def order_by(self, *clauses):
        """return a new selectable with the given list of ORDER BY criterion applied.

        The criterion will be appended to any pre-existing ORDER BY criterion.

        """
        self.append_order_by(*clauses)

    @_generative
    def group_by(self, *clauses):
        """return a new selectable with the given list of GROUP BY criterion applied.

        The criterion will be appended to any pre-existing GROUP BY criterion.

        """
        self.append_group_by(*clauses)

    def append_order_by(self, *clauses):
        """Append the given ORDER BY criterion applied to this selectable.

        The criterion will be appended to any pre-existing ORDER BY criterion.

        """
        if len(clauses) == 1 and clauses[0] is None:
            self._order_by_clause = ClauseList()
        else:
            if getattr(self, '_order_by_clause', None):
                clauses = list(self._order_by_clause) + list(clauses)
            self._order_by_clause = ClauseList(*clauses)

    def append_group_by(self, *clauses):
        """Append the given GROUP BY criterion applied to this selectable.

        The criterion will be appended to any pre-existing GROUP BY criterion.

        """
        if len(clauses) == 1 and clauses[0] is None:
            self._group_by_clause = ClauseList()
        else:
            if getattr(self, '_group_by_clause', None):
                clauses = list(self._group_by_clause) + list(clauses)
            self._group_by_clause = ClauseList(*clauses)

    @property
    def _from_objects(self):
        return [self]


class _ScalarSelect(_Grouping):
    _from_objects = []

    def __init__(self, element):
        self.element = element
        cols = list(element.c)
        if len(cols) != 1:
            raise exc.InvalidRequestError("Scalar select can only be created "
                    "from a Select object that has exactly one column expression.")
        self.type = cols[0].type

    @property
    def columns(self):
        raise exc.InvalidRequestError("Scalar Select expression has no columns; "
                    "use this object directly within a column-level expression.")
    c  = columns

    def self_group(self, **kwargs):
        return self

    def _make_proxy(self, selectable, name):
        return list(self.inner_columns)[0]._make_proxy(selectable, name)

class CompoundSelect(_SelectBaseMixin, FromClause):
    """Forms the basis of ``UNION``, ``UNION ALL``, and other SELECT-based set operations."""

    __visit_name__ = 'compound_select'

    def __init__(self, keyword, *selects, **kwargs):
        self._should_correlate = kwargs.pop('correlate', False)
        self.keyword = keyword
        self.selects = []

        numcols = None

        # some DBs do not like ORDER BY in the inner queries of a UNION, etc.
        for n, s in enumerate(selects):
            s = _clause_element_as_expr(s)
            
            if not numcols:
                numcols = len(s.c)
            elif len(s.c) != numcols:
                raise exc.ArgumentError(
                        "All selectables passed to CompoundSelect must "
                        "have identical numbers of columns; select #%d has %d columns, select #%d has %d" %
                        (1, len(self.selects[0].c), n+1, len(s.c))
                )

            # unions group from left to right, so don't group first select
            if n:
                self.selects.append(s.self_group(self))
            else:
                self.selects.append(s)

        _SelectBaseMixin.__init__(self, **kwargs)

    def self_group(self, against=None):
        return _FromGrouping(self)

    def is_derived_from(self, fromclause):
        for s in self.selects:
            if s.is_derived_from(fromclause):
                return True
        return False

    def _populate_column_collection(self):
        for cols in zip(*[s.c for s in self.selects]):
            # this is a slightly hacky thing - the union exports a column that
            # resembles just that of the *first* selectable.  to get at a "composite" column,
            # particularly foreign keys, you have to dig through the proxies collection 
            # which we generate below.  We may want to improve upon this,
            # such as perhaps _make_proxy can accept a list of other columns that
            # are "shared" - schema.column can then copy all the ForeignKeys in.
            # this would allow the union() to have all those fks too.
            proxy = cols[0]._make_proxy(
                            self, name=self.use_labels and cols[0]._label or None)
            
            # hand-construct the "proxies" collection to include all derived columns
            # place a 'weight' annotation corresponding to how low in the list of
            # select()s the column occurs, so that the corresponding_column() operation
            # can resolve conflicts
            proxy.proxies = [c._annotate({'weight':i + 1}) for i, c in enumerate(cols)]
            
    def _copy_internals(self, clone=_clone):
        self._reset_exported()
        self.selects = [clone(s) for s in self.selects]
        if hasattr(self, '_col_map'):
            del self._col_map
        for attr in ('_order_by_clause', '_group_by_clause'):
            if getattr(self, attr) is not None:
                setattr(self, attr, clone(getattr(self, attr)))

    def get_children(self, column_collections=True, **kwargs):
        return (column_collections and list(self.c) or []) + \
            [self._order_by_clause, self._group_by_clause] + list(self.selects)

    def bind(self):
        if self._bind:
            return self._bind
        for s in self.selects:
            e = s.bind
            if e:
                return e
        else:
            return None
    def _set_bind(self, bind):
        self._bind = bind
    bind = property(bind, _set_bind)

class Select(_SelectBaseMixin, FromClause):
    """Represents a ``SELECT`` statement.

    Select statements support appendable clauses, as well as the
    ability to execute themselves and return a result set.

    """

    __visit_name__ = 'select'

    def __init__(self, columns, whereclause=None, from_obj=None, distinct=False, having=None, correlate=True, prefixes=None, **kwargs):
        """Construct a Select object.

        The public constructor for Select is the
        :func:`~sqlalchemy.sql.expression.select` function; see that function for
        argument descriptions.

        Additional generative and mutator methods are available on the
        :class:`~sqlalchemy.sql.expression._SelectBaseMixin` superclass.

        """
        self._should_correlate = correlate
        self._distinct = distinct

        self._correlate = set()
        self._froms = util.OrderedSet()

        if columns:
            self._raw_columns = [
                isinstance(c, _ScalarSelect) and c.self_group(against=operators.comma_op) or c
                for c in
                [_literal_as_column(c) for c in columns]
            ]

            self._froms.update(_from_objects(*self._raw_columns))
        else:
            self._raw_columns = []

        if whereclause:
            self._whereclause = _literal_as_text(whereclause)
            self._froms.update(_from_objects(self._whereclause))
        else:
            self._whereclause = None

        if from_obj:
            self._froms.update(
                _is_literal(f) and _TextClause(f) or f
                for f in util.to_list(from_obj))

        if having:
            self._having = _literal_as_text(having)
        else:
            self._having = None

        if prefixes:
            self._prefixes = [_literal_as_text(p) for p in prefixes]
        else:
            self._prefixes = []

        _SelectBaseMixin.__init__(self, **kwargs)

    def _get_display_froms(self, existing_froms=None):
        """Return the full list of 'from' clauses to be displayed.

        Takes into account a set of existing froms which may be
        rendered in the FROM clause of enclosing selects; this Select
        may want to leave those absent if it is automatically
        correlating.

        """
        froms = self._froms

        toremove = itertools.chain(*[f._hide_froms for f in froms])
        if toremove:
            froms = froms.difference(toremove)

        if len(froms) > 1 or self._correlate:
            if self._correlate:
                froms = froms.difference(_cloned_intersection(froms, self._correlate))

            if self._should_correlate and existing_froms:
                froms = froms.difference(_cloned_intersection(froms, existing_froms))

                if not len(froms):
                    raise exc.InvalidRequestError(
                            "Select statement '%s' returned no FROM clauses "
                            "due to auto-correlation; specify correlate(<tables>) "
                            "to control correlation manually." % self)

        return froms

    @property
    def froms(self):
        """Return the displayed list of FromClause elements."""

        return self._get_display_froms()

    @property
    def type(self):
        raise exc.InvalidRequestError("Select objects don't have a type.  "
                    "Call as_scalar() on this Select object "
                    "to return a 'scalar' version of this Select.")

    @util.memoized_instancemethod
    def locate_all_froms(self):
        """return a Set of all FromClause elements referenced by this Select.

        This set is a superset of that returned by the ``froms`` property, which
        is specifically for those FromClause elements that would actually be rendered.

        """
        return self._froms.union(_from_objects(*list(self._froms)))

    @property
    def inner_columns(self):
        """an iterator of all ColumnElement expressions which would
        be rendered into the columns clause of the resulting SELECT statement.

        """

        return itertools.chain(*[c._select_iterable for c in self._raw_columns])

    def is_derived_from(self, fromclause):
        if self in fromclause._cloned_set:
            return True

        for f in self.locate_all_froms():
            if f.is_derived_from(fromclause):
                return True
        return False

    def _copy_internals(self, clone=_clone):
        self._reset_exported()
        from_cloned = dict((f, clone(f))
                           for f in self._froms.union(self._correlate))
        self._froms = set(from_cloned[f] for f in self._froms)
        self._correlate = set(from_cloned[f] for f in self._correlate)
        self._raw_columns = [clone(c) for c in self._raw_columns]
        for attr in ('_whereclause', '_having', '_order_by_clause', '_group_by_clause'):
            if getattr(self, attr) is not None:
                setattr(self, attr, clone(getattr(self, attr)))

    def get_children(self, column_collections=True, **kwargs):
        """return child elements as per the ClauseElement specification."""

        return (column_collections and list(self.columns) or []) + \
            self._raw_columns + list(self._froms) + \
            [x for x in (self._whereclause, self._having, self._order_by_clause, self._group_by_clause) if x is not None]

    @_generative
    def column(self, column):
        """return a new select() construct with the given column expression added to its columns clause."""

        column = _literal_as_column(column)

        if isinstance(column, _ScalarSelect):
            column = column.self_group(against=operators.comma_op)

        self._raw_columns = self._raw_columns + [column]
        self._froms = self._froms.union(_from_objects(column))

    @_generative
    def with_only_columns(self, columns):
        """return a new select() construct with its columns clause replaced with the given columns."""

        self._raw_columns = [
                isinstance(c, _ScalarSelect) and c.self_group(against=operators.comma_op) or c
                for c in
                [_literal_as_column(c) for c in columns]
            ]

    @_generative
    def where(self, whereclause):
        """return a new select() construct with the given expression added to its WHERE clause, joined
        to the existing clause via AND, if any."""

        self.append_whereclause(whereclause)

    @_generative
    def having(self, having):
        """return a new select() construct with the given expression added to its HAVING clause, joined
        to the existing clause via AND, if any."""

        self.append_having(having)

    @_generative
    def distinct(self):
        """return a new select() construct which will apply DISTINCT to its columns clause."""

        self._distinct = True

    @_generative
    def prefix_with(self, clause):
        """return a new select() construct which will apply the given expression to the start of its
        columns clause, not using any commas."""

        clause = _literal_as_text(clause)
        self._prefixes = self._prefixes + [clause]

    @_generative
    def select_from(self, fromclause):
        """return a new select() construct with the given FROM expression applied to its list of
        FROM objects."""

        fromclause = _literal_as_text(fromclause)
        self._froms = self._froms.union([fromclause])

    @_generative
    def correlate(self, *fromclauses):
        """return a new select() construct which will correlate the given FROM clauses to that
        of an enclosing select(), if a match is found.

        By "match", the given fromclause must be present in this select's list of FROM objects
        and also present in an enclosing select's list of FROM objects.

        Calling this method turns off the select's default behavior of "auto-correlation".  Normally,
        select() auto-correlates all of its FROM clauses to those of an embedded select when
        compiled.

        If the fromclause is None, correlation is disabled for the returned select().

        """
        self._should_correlate = False
        if fromclauses == (None,):
            self._correlate = set()
        else:
            self._correlate = self._correlate.union(fromclauses)

    def append_correlation(self, fromclause):
        """append the given correlation expression to this select() construct."""

        self._should_correlate = False
        self._correlate = self._correlate.union([fromclause])

    def append_column(self, column):
        """append the given column expression to the columns clause of this select() construct."""

        column = _literal_as_column(column)

        if isinstance(column, _ScalarSelect):
            column = column.self_group(against=operators.comma_op)

        self._raw_columns = self._raw_columns + [column]
        self._froms = self._froms.union(_from_objects(column))
        self._reset_exported()

    def append_prefix(self, clause):
        """append the given columns clause prefix expression to this select() construct."""

        clause = _literal_as_text(clause)
        self._prefixes = self._prefixes.union([clause])

    def append_whereclause(self, whereclause):
        """append the given expression to this select() construct's WHERE criterion.

        The expression will be joined to existing WHERE criterion via AND.

        """
        whereclause = _literal_as_text(whereclause)
        self._froms = self._froms.union(_from_objects(whereclause))

        if self._whereclause is not None:
            self._whereclause = and_(self._whereclause, whereclause)
        else:
            self._whereclause = whereclause

    def append_having(self, having):
        """append the given expression to this select() construct's HAVING criterion.

        The expression will be joined to existing HAVING criterion via AND.

        """
        if self._having is not None:
            self._having = and_(self._having, _literal_as_text(having))
        else:
            self._having = _literal_as_text(having)

    def append_from(self, fromclause):
        """append the given FromClause expression to this select() construct's FROM clause.

        """
        if _is_literal(fromclause):
            fromclause = _TextClause(fromclause)

        self._froms = self._froms.union([fromclause])

    def __exportable_columns(self):
        for column in self._raw_columns:
            if isinstance(column, Selectable):
                for co in column.columns:
                    yield co
            elif isinstance(column, ColumnElement):
                yield column
            else:
                continue

    def _populate_column_collection(self):
        for c in self.__exportable_columns():
            c._make_proxy(self, name=self.use_labels and c._label or None)

    def self_group(self, against=None):
        """return a 'grouping' construct as per the ClauseElement specification.

        This produces an element that can be embedded in an expression.  Note that
        this method is called automatically as needed when constructing expressions.

        """
        if isinstance(against, CompoundSelect):
            return self
        return _FromGrouping(self)

    def union(self, other, **kwargs):
        """return a SQL UNION of this select() construct against the given selectable."""

        return union(self, other, **kwargs)

    def union_all(self, other, **kwargs):
        """return a SQL UNION ALL of this select() construct against the given selectable."""

        return union_all(self, other, **kwargs)

    def except_(self, other, **kwargs):
        """return a SQL EXCEPT of this select() construct against the given selectable."""

        return except_(self, other, **kwargs)

    def except_all(self, other, **kwargs):
        """return a SQL EXCEPT ALL of this select() construct against the given selectable."""

        return except_all(self, other, **kwargs)

    def intersect(self, other, **kwargs):
        """return a SQL INTERSECT of this select() construct against the given selectable."""

        return intersect(self, other, **kwargs)

    def intersect_all(self, other, **kwargs):
        """return a SQL INTERSECT ALL of this select() construct against the given selectable."""

        return intersect_all(self, other, **kwargs)

    def bind(self):
        if self._bind:
            return self._bind
        if not self._froms:
            for c in self._raw_columns:
                e = c.bind
                if e:
                    self._bind = e
                    return e
        else:
            e = list(self._froms)[0].bind
            if e:
                self._bind = e
                return e

        return None

    def _set_bind(self, bind):
        self._bind = bind
    bind = property(bind, _set_bind)

class _UpdateBase(ClauseElement):
    """Form the base for ``INSERT``, ``UPDATE``, and ``DELETE`` statements."""

    __visit_name__ = 'update_base'

    supports_execution = True
    _autocommit = True

    def _generate(self):
        s = self.__class__.__new__(self.__class__)
        s.__dict__ = self.__dict__.copy()
        return s

    def _process_colparams(self, parameters):
        if isinstance(parameters, (list, tuple)):
            pp = {}
            for i, c in enumerate(self.table.c):
                pp[c.key] = parameters[i]
            return pp
        else:
            return parameters

    def params(self, *arg, **kw):
        raise NotImplementedError("params() is not supported for INSERT/UPDATE/DELETE statements."
            "  To set the values for an INSERT or UPDATE statement, use stmt.values(**parameters).")

    def bind(self):
        return self._bind or self.table.bind

    def _set_bind(self, bind):
        self._bind = bind
    bind = property(bind, _set_bind)

class _ValuesBase(_UpdateBase):

    __visit_name__ = 'values_base'

    def __init__(self, table, values):
        self.table = table
        self.parameters = self._process_colparams(values)

    @_generative
    def values(self, *args, **kwargs):
        """specify the VALUES clause for an INSERT statement, or the SET clause for an UPDATE.

            \**kwargs
                key=<somevalue> arguments

            \*args
                A single dictionary can be sent as the first positional argument.  This allows
                non-string based keys, such as Column objects, to be used.

        """
        if args:
            v = args[0]
        else:
            v = {}

        if self.parameters is None:
            self.parameters = self._process_colparams(v)
            self.parameters.update(kwargs)
        else:
            self.parameters = self.parameters.copy()
            self.parameters.update(self._process_colparams(v))
            self.parameters.update(kwargs)

class Insert(_ValuesBase):
    """Represent an INSERT construct.

    The ``Insert`` object is created using the :func:`insert()` function.

    """
    __visit_name__ = 'insert'

    def __init__(self, table, values=None, inline=False, bind=None, prefixes=None, **kwargs):
        _ValuesBase.__init__(self, table, values)
        self._bind = bind
        self.select = None
        self.inline = inline
        if prefixes:
            self._prefixes = [_literal_as_text(p) for p in prefixes]
        else:
            self._prefixes = []
        self.kwargs = kwargs

    def get_children(self, **kwargs):
        if self.select is not None:
            return self.select,
        else:
            return ()

    def _copy_internals(self, clone=_clone):
        # TODO: coverage
        self.parameters = self.parameters.copy()

    @_generative
    def prefix_with(self, clause):
        """Add a word or expression between INSERT and INTO. Generative.

        If multiple prefixes are supplied, they will be separated with
        spaces.

        """
        clause = _literal_as_text(clause)
        self._prefixes = self._prefixes + [clause]

class Update(_ValuesBase):
    """Represent an Update construct.

    The ``Update`` object is created using the :func:`update()` function.

    """
    __visit_name__ = 'update'

    def __init__(self, table, whereclause, values=None, inline=False, bind=None, **kwargs):
        _ValuesBase.__init__(self, table, values)
        self._bind = bind
        if whereclause:
            self._whereclause = _literal_as_text(whereclause)
        else:
            self._whereclause = None
        self.inline = inline
        self.kwargs = kwargs

    def get_children(self, **kwargs):
        if self._whereclause is not None:
            return self._whereclause,
        else:
            return ()

    def _copy_internals(self, clone=_clone):
        # TODO: coverage
        self._whereclause = clone(self._whereclause)
        self.parameters = self.parameters.copy()

    @_generative
    def where(self, whereclause):
        """return a new update() construct with the given expression added to its WHERE clause, joined
        to the existing clause via AND, if any."""

        if self._whereclause is not None:
            self._whereclause = and_(self._whereclause, _literal_as_text(whereclause))
        else:
            self._whereclause = _literal_as_text(whereclause)


class Delete(_UpdateBase):
    """Represent a DELETE construct.

    The ``Delete`` object is created using the :func:`delete()` function.

    """

    __visit_name__ = 'delete'

    def __init__(self, table, whereclause, bind=None, **kwargs):
        self._bind = bind
        self.table = table
        if whereclause:
            self._whereclause = _literal_as_text(whereclause)
        else:
            self._whereclause = None

        self.kwargs = kwargs

    def get_children(self, **kwargs):
        if self._whereclause is not None:
            return self._whereclause,
        else:
            return ()

    @_generative
    def where(self, whereclause):
        """Add the given WHERE clause to a newly returned delete construct."""

        if self._whereclause is not None:
            self._whereclause = and_(self._whereclause, _literal_as_text(whereclause))
        else:
            self._whereclause = _literal_as_text(whereclause)

    def _copy_internals(self, clone=_clone):
        # TODO: coverage
        self._whereclause = clone(self._whereclause)

class _IdentifiedClause(ClauseElement):
    __visit_name__ = 'identified'
    supports_execution = True
    _autocommit = False
    quote = None

    def __init__(self, ident):
        self.ident = ident

class SavepointClause(_IdentifiedClause):
    __visit_name__ = 'savepoint'

class RollbackToSavepointClause(_IdentifiedClause):
    __visit_name__ = 'rollback_to_savepoint'

class ReleaseSavepointClause(_IdentifiedClause):
    __visit_name__ = 'release_savepoint'