SQL Expressions
=================

.. contents::
    :local:
    :class: faq
    :backlinks: none

.. _faq_sql_expression_string:

How do I render SQL expressions as strings, possibly with bound parameters inlined?
------------------------------------------------------------------------------------

The "stringification" of a SQLAlchemy statement or Query in the vast majority
of cases is as simple as::

    print(str(statement))

this applies both to an ORM :class:`~.orm.query.Query` as well as any :func:`.select` or other
statement.   Additionally, to get the statement as compiled to a
specific dialect or engine, if the statement itself is not already
bound to one you can pass this in to :meth:`.ClauseElement.compile`::

    print(statement.compile(someengine))

or without an :class:`.Engine`::

    from sqlalchemy.dialects import postgresql
    print(statement.compile(dialect=postgresql.dialect()))

When given an ORM :class:`~.orm.query.Query` object, in order to get at the
:meth:`.ClauseElement.compile`
method we only need access the :attr:`~.orm.query.Query.statement`
accessor first::

    statement = query.statement
    print(statement.compile(someengine))

The above forms will render the SQL statement as it is passed to the Python
:term:`DBAPI`, which includes that bound parameters are not rendered inline.
SQLAlchemy normally does not stringify bound parameters, as this is handled
appropriately by the Python DBAPI, not to mention bypassing bound
parameters is probably the most widely exploited security hole in
modern web applications.   SQLAlchemy has limited ability to do this
stringification in certain circumstances such as that of emitting DDL.
In order to access this functionality one can use the ``literal_binds``
flag, passed to ``compile_kwargs``::

    from sqlalchemy.sql import table, column, select

    t = table('t', column('x'))

    s = select([t]).where(t.c.x == 5)

    print(s.compile(compile_kwargs={"literal_binds": True}))

the above approach has the caveats that it is only supported for basic
types, such as ints and strings, and furthermore if a :func:`.bindparam`
without a pre-set value is used directly, it won't be able to
stringify that either.

To support inline literal rendering for types not supported, implement
a :class:`.TypeDecorator` for the target type which includes a
:meth:`.TypeDecorator.process_literal_param` method::

    from sqlalchemy import TypeDecorator, Integer


    class MyFancyType(TypeDecorator):
        impl = Integer

        def process_literal_param(self, value, dialect):
            return "my_fancy_formatting(%s)" % value

    from sqlalchemy import Table, Column, MetaData

    tab = Table('mytable', MetaData(), Column('x', MyFancyType()))

    print(
        tab.select().where(tab.c.x > 5).compile(
            compile_kwargs={"literal_binds": True})
    )

producing output like::

    SELECT mytable.x
    FROM mytable
    WHERE mytable.x > my_fancy_formatting(5)


Why does ``.col.in_([])`` Produce ``col != col``? Why not ``1=0``?
-------------------------------------------------------------------

A little introduction to the issue. The IN operator in SQL, given a list of
elements to compare against a column, generally does not accept an empty list,
that is while it is valid to say::

    column IN (1, 2, 3)

it's not valid to say::

    column IN ()

SQLAlchemy's :meth:`.Operators.in_` operator, when given an empty list, produces this
expression::

    column != column

As of version 0.6, it also produces a warning stating that a less efficient
comparison operation will be rendered. This expression is the only one that is
both database agnostic and produces correct results.

For example, the naive approach of "just evaluate to false, by comparing 1=0
or 1!=1", does not handle nulls properly. An expression like::

    NOT column != column

will not return a row when "column" is null, but an expression which does not
take the column into account::

    NOT 1=0

will.

Closer to the mark is the following CASE expression::

    CASE WHEN column IS NOT NULL THEN 1=0 ELSE NULL END

We don't use this expression due to its verbosity, and its also not
typically accepted by Oracle within a WHERE clause - depending
on how you phrase it, you'll either get "ORA-00905: missing keyword" or
"ORA-00920: invalid relational operator". It's also still less efficient than
just rendering SQL without the clause altogether (or not issuing the SQL at
all, if the statement is just a simple search).

The best approach therefore is to avoid the usage of IN given an argument list
of zero length.  Instead, don't emit the Query in the first place, if no rows
should be returned.  The warning is best promoted to a full error condition
using the Python warnings filter (see http://docs.python.org/library/warnings.html).