django/docs/topics/db/sql.txt

==========================
Performing raw SQL queries
==========================

.. currentmodule:: django.db.models

When the :doc:`model query APIs </topics/db/queries>` don't go far enough, you
can fall back to writing raw SQL. Django gives you two ways of performing raw
SQL queries: you can use :meth:`Manager.raw()` to `perform raw queries and
return model instances`__, or you can avoid the model layer entirely and
`execute custom SQL directly`__.

__ `performing raw queries`_
__ `executing custom SQL directly`_

.. warning::

    You should be very careful whenever you write raw SQL. Every time you use
    it, you should properly escape any parameters that the user can control
    by using ``params`` in order to protect against SQL injection attacks.
    Please read more about :ref:`SQL injection protection
    <sql-injection-protection>`.

.. _executing-raw-queries:

Performing raw queries
======================

The ``raw()`` manager method can be used to perform raw SQL queries that
return model instances:

.. method:: Manager.raw(raw_query, params=None, translations=None)

This method takes a raw SQL query, executes it, and returns a
``django.db.models.query.RawQuerySet`` instance. This ``RawQuerySet`` instance
can be iterated over just like a normal
:class:`~django.db.models.query.QuerySet` to provide object instances.

This is best illustrated with an example. Suppose you have the following model::

    class Person(models.Model):
        first_name = models.CharField(...)
        last_name = models.CharField(...)
        birth_date = models.DateField(...)

You could then execute custom SQL like so::

    >>> for p in Person.objects.raw('SELECT * FROM myapp_person'):
    ...     print(p)
    John Smith
    Jane Jones

Of course, this example isn't very exciting -- it's exactly the same as
running ``Person.objects.all()``. However, ``raw()`` has a bunch of other
options that make it very powerful.

.. admonition:: Model table names

    Where did the name of the ``Person`` table come from in that example?

    By default, Django figures out a database table name by joining the
    model's "app label" -- the name you used in ``manage.py startapp`` -- to
    the model's class name, with an underscore between them. In the example
    we've assumed that the ``Person`` model lives in an app named ``myapp``,
    so its table would be ``myapp_person``.

    For more details check out the documentation for the
    :attr:`~Options.db_table` option, which also lets you manually set the
    database table name.

.. warning::

    No checking is done on the SQL statement that is passed in to ``.raw()``.
    Django expects that the statement will return a set of rows from the
    database, but does nothing to enforce that. If the query does not
    return rows, a (possibly cryptic) error will result.

.. warning::

    If you are performing queries on MySQL, note that MySQL's silent type coercion
    may cause unexpected results when mixing types. If you query on a string
    type column, but with an integer value, MySQL will coerce the types of all values
    in the table to an integer before performing the comparison. For example, if your
    table contains the values ``'abc'``, ``'def'`` and you query for ``WHERE mycolumn=0``,
    both rows will match. To prevent this, perform the correct typecasting
    before using the value in a query.

.. warning::

    While a ``RawQuerySet`` instance can be iterated over like a normal
    :class:`~django.db.models.query.QuerySet`, ``RawQuerySet`` doesn't
    implement all methods you can use with ``QuerySet``. For example,
    ``__bool__()`` and ``__len__()`` are not defined in ``RawQuerySet``, and
    thus all ``RawQuerySet`` instances are considered ``True``. The reason
    these methods are not implemented in ``RawQuerySet`` is that implementing
    them without internal caching would be a performance drawback and adding
    such caching would be backward incompatible.

Mapping query fields to model fields
------------------------------------

``raw()`` automatically maps fields in the query to fields on the model.

The order of fields in your query doesn't matter. In other words, both
of the following queries work identically::

    >>> Person.objects.raw('SELECT id, first_name, last_name, birth_date FROM myapp_person')
    ...
    >>> Person.objects.raw('SELECT last_name, birth_date, first_name, id FROM myapp_person')
    ...

Matching is done by name. This means that you can use SQL's ``AS`` clauses to
map fields in the query to model fields. So if you had some other table that
had ``Person`` data in it, you could easily map it into ``Person`` instances::

    >>> Person.objects.raw('''SELECT first AS first_name,
    ...                              last AS last_name,
    ...                              bd AS birth_date,
    ...                              pk AS id,
    ...                       FROM some_other_table''')

As long as the names match, the model instances will be created correctly.

Alternatively, you can map fields in the query to model fields using the
``translations`` argument to ``raw()``. This is a dictionary mapping names of
fields in the query to names of fields on the model. For example, the above
query could also be written::

    >>> name_map = {'first': 'first_name', 'last': 'last_name', 'bd': 'birth_date', 'pk': 'id'}
    >>> Person.objects.raw('SELECT * FROM some_other_table', translations=name_map)

Index lookups
-------------

``raw()`` supports indexing, so if you need only the first result you can
write::

    >>> first_person = Person.objects.raw('SELECT * FROM myapp_person')[0]

However, the indexing and slicing are not performed at the database level. If
you have a large number of ``Person`` objects in your database, it is more
efficient to limit the query at the SQL level::

    >>> first_person = Person.objects.raw('SELECT * FROM myapp_person LIMIT 1')[0]

Deferring model fields
----------------------

Fields may also be left out::

    >>> people = Person.objects.raw('SELECT id, first_name FROM myapp_person')

The ``Person`` objects returned by this query will be deferred model instances
(see :meth:`~django.db.models.query.QuerySet.defer()`). This means that the
fields that are omitted from the query will be loaded on demand. For example::

    >>> for p in Person.objects.raw('SELECT id, first_name FROM myapp_person'):
    ...     print(p.first_name, # This will be retrieved by the original query
    ...           p.last_name) # This will be retrieved on demand
    ...
    John Smith
    Jane Jones

From outward appearances, this looks like the query has retrieved both
the first name and last name. However, this example actually issued 3
queries. Only the first names were retrieved by the raw() query -- the
last names were both retrieved on demand when they were printed.

There is only one field that you can't leave out - the primary key
field. Django uses the primary key to identify model instances, so it
must always be included in a raw query. An ``InvalidQuery`` exception
will be raised if you forget to include the primary key.

Adding annotations
------------------

You can also execute queries containing fields that aren't defined on the
model. For example, we could use `PostgreSQL's age() function`__ to get a list
of people with their ages calculated by the database::

    >>> people = Person.objects.raw('SELECT *, age(birth_date) AS age FROM myapp_person')
    >>> for p in people:
    ...     print("%s is %s." % (p.first_name, p.age))
    John is 37.
    Jane is 42.
    ...

__ http://www.postgresql.org/docs/current/static/functions-datetime.html

Passing parameters into ``raw()``
---------------------------------

If you need to perform parameterized queries, you can use the ``params``
argument to ``raw()``::

    >>> lname = 'Doe'
    >>> Person.objects.raw('SELECT * FROM myapp_person WHERE last_name = %s', [lname])

``params`` is a list or dictionary of parameters. You'll use ``%s``
placeholders in the query string for a list, or ``%(key)s``
placeholders for a dictionary (where ``key`` is replaced by a
dictionary key, of course), regardless of your database engine.  Such
placeholders will be replaced with parameters from the ``params``
argument.

.. note::

   Dictionary params are not supported with the SQLite backend; with
   this backend, you must pass parameters as a list.

.. warning::

    **Do not use string formatting on raw queries!**

    It's tempting to write the above query as::

        >>> query = 'SELECT * FROM myapp_person WHERE last_name = %s' % lname
        >>> Person.objects.raw(query)

    **Don't.**

    Using the ``params`` argument completely protects you from `SQL injection
    attacks`__, a common exploit where attackers inject arbitrary SQL into
    your database. If you use string interpolation, sooner or later you'll
    fall victim to SQL injection. As long as you remember to always use the
    ``params`` argument you'll be protected.

__ https://en.wikipedia.org/wiki/SQL_injection

.. _executing-custom-sql:

Executing custom SQL directly
=============================

Sometimes even :meth:`Manager.raw` isn't quite enough: you might need to
perform queries that don't map cleanly to models, or directly execute
``UPDATE``, ``INSERT``, or ``DELETE`` queries.

In these cases, you can always access the database directly, routing around
the model layer entirely.

The object ``django.db.connection`` represents the default database
connection. To use the database connection, call ``connection.cursor()`` to
get a cursor object. Then, call ``cursor.execute(sql, [params])`` to execute
the SQL and ``cursor.fetchone()`` or ``cursor.fetchall()`` to return the
resulting rows.

For example::

    from django.db import connection

    def my_custom_sql(self):
        with connection.cursor() as cursor:
            cursor.execute("UPDATE bar SET foo = 1 WHERE baz = %s", [self.baz])
            cursor.execute("SELECT foo FROM bar WHERE baz = %s", [self.baz])
            row = cursor.fetchone()

        return row

Note that if you want to include literal percent signs in the query, you have to
double them in the case you are passing parameters::

     cursor.execute("SELECT foo FROM bar WHERE baz = '30%'")
     cursor.execute("SELECT foo FROM bar WHERE baz = '30%%' AND id = %s", [self.id])

If you are using :doc:`more than one database </topics/db/multi-db>`, you can
use ``django.db.connections`` to obtain the connection (and cursor) for a
specific database. ``django.db.connections`` is a dictionary-like
object that allows you to retrieve a specific connection using its
alias::

    from django.db import connections
    cursor = connections['my_db_alias'].cursor()
    # Your code here...

By default, the Python DB API will return results without their field names,
which means you end up with a ``list`` of values, rather than a ``dict``. At a
small performance and memory cost, you can return results as a ``dict`` by
using something like this::

    def dictfetchall(cursor):
        "Return all rows from a cursor as a dict"
        columns = [col[0] for col in cursor.description]
        return [
            dict(zip(columns, row))
            for row in cursor.fetchall()
        ]

Another option is to use :func:`collections.namedtuple` from the Python
standard library. A ``namedtuple`` is a tuple-like object that has fields
accessible by attribute lookup; it's also indexable and iterable. Results are
immutable and accessible by field names or indices, which might be useful::

    from collections import namedtuple

    def namedtuplefetchall(cursor):
        "Return all rows from a cursor as a namedtuple"
        desc = cursor.description
        nt_result = namedtuple('Result', [col[0] for col in desc])
        return [nt_result(*row) for row in cursor.fetchall()]

Here is an example of the difference between the three::

    >>> cursor.execute("SELECT id, parent_id FROM test LIMIT 2");
    >>> cursor.fetchall()
    ((54360982, None), (54360880, None))

    >>> cursor.execute("SELECT id, parent_id FROM test LIMIT 2");
    >>> dictfetchall(cursor)
    [{'parent_id': None, 'id': 54360982}, {'parent_id': None, 'id': 54360880}]

    >>> cursor.execute("SELECT id, parent_id FROM test LIMIT 2");
    >>> results = namedtuplefetchall(cursor)
    >>> results
    [Result(id=54360982, parent_id=None), Result(id=54360880, parent_id=None)]
    >>> results[0].id
    54360982
    >>> results[0][0]
    54360982

Connections and cursors
-----------------------

``connection`` and ``cursor`` mostly implement the standard Python DB-API
described in :pep:`249` — except when it comes to :doc:`transaction handling
</topics/db/transactions>`.

If you're not familiar with the Python DB-API, note that the SQL statement in
``cursor.execute()`` uses placeholders, ``"%s"``, rather than adding
parameters directly within the SQL. If you use this technique, the underlying
database library will automatically escape your parameters as necessary.

Also note that Django expects the ``"%s"`` placeholder, *not* the ``"?"``
placeholder, which is used by the SQLite Python bindings. This is for the sake
of consistency and sanity.

Using a cursor as a context manager::

    with connection.cursor() as c:
        c.execute(...)

is equivalent to::

    c = connection.cursor()
    try:
        c.execute(...)
    finally:
        c.close()