elements.py 文件源码

def _create_distinct(cls, expr):
        """Produce an column-expression-level unary ``DISTINCT`` clause.

        This applies the ``DISTINCT`` keyword to an individual column
        expression, and is typically contained within an aggregate function,
        as in::

            from sqlalchemy import distinct, func
            stmt = select([func.count(distinct(users_table.c.name))])

        The above would produce an expression resembling::

            SELECT COUNT(DISTINCT name) FROM user

        The :func:`.distinct` function is also available as a column-level
        method, e.g. :meth:`.ColumnElement.distinct`, as in::

            stmt = select([func.count(users_table.c.name.distinct())])

        The :func:`.distinct` operator is different from the
        :meth:`.Select.distinct` method of :class:`.Select`,
        which produces a ``SELECT`` statement
        with ``DISTINCT`` applied to the result set as a whole,
        e.g. a ``SELECT DISTINCT`` expression.  See that method for further
        information.

        .. seealso::

            :meth:`.ColumnElement.distinct`

            :meth:`.Select.distinct`

            :data:`.func`

        """
        expr = _literal_as_binds(expr)
        return UnaryExpression(
            expr, operator=operators.distinct_op,
            type_=expr.type, wraps_column_expression=False)