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Edited doc changes from [17244] 

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17302 bcc190cf-cafb-0310-a4f2-bffc1f526a37
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Adrian Holovaty 2011-12-30 18:01:34 +00:00
parent afb21094a9
commit d3ef13044e
3 changed files with 20 additions and 16 deletions
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26
docs/ref/models/querysets.txt
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@ -376,20 +376,26 @@ query spans multiple tables, it's possible to get duplicate results when a
.. versionadded:: 1.4
The possibility to pass positional arguments (``*fields``) is new in Django 1.4.
They are names of fields to which the ``DISTINCT`` should be limited. This
translates to a ``SELECT DISTINCT ON`` SQL query. A ``DISTINCT ON`` query eliminates
duplicate rows not by comparing all fields in a row, but by comparing only the given
fields.
As of Django 1.4, you can pass positional arguments (``*fields``) in order to
specify the names of fields to which the ``DISTINCT`` should apply. This
translates to a ``SELECT DISTINCT ON`` SQL query.
Here's the difference. For a normal ``distinct()`` call, the database compares
*each* field in each row when determining which rows are distinct. For a
``distinct()`` call with specified field names, the database will only compare
the specified field names.
.. note::
Note that the ability to specify field names is only available in PostgreSQL.
This ability to specify field names is only available in PostgreSQL.
.. note::
When using the ``DISTINCT ON`` functionality it is required that the columns given
to :meth:`distinct` match the first :meth:`order_by` columns. For example ``SELECT
DISTINCT ON (a)`` gives you the first row for each value in column ``a``. If you
don't specify an order, then you'll get some arbitrary row.
When you specify field names, you *must* provide an ``order_by()`` in the
QuerySet, and the fields in ``order_by()`` must start with the fields in
``distinct()``, in the same order.
For example, ``SELECT DISTINCT ON (a)`` gives you the first row for each
value in column ``a``. If you don't specify an order, you'll get some
arbitrary row.
Examples::

5
docs/releases/1.4-alpha-1.txt
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@ -548,10 +548,9 @@ Django 1.4 also includes several smaller improvements worth noting:
* Added support in the ORM for generating ``SELECT`` queries containing
``DISTINCT ON``.
The ``distinct()`` ``Queryset`` method now accepts an optional list of model
The ``distinct()`` ``QuerySet`` method now accepts an optional list of model
field names. If specified, then the ``DISTINCT`` statement is limited to these
fields. PostgreSQL is the only database backend shipped with Django that
supports this new functionality.
fields. This is only supported in PostgreSQL.
For more details, see the documentation for
:meth:`~django.db.models.query.QuerySet.distinct`.

5
docs/releases/1.4.txt
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@ -538,10 +538,9 @@ Django 1.4 also includes several smaller improvements worth noting:
* Added support in the ORM for generating ``SELECT`` queries containing
``DISTINCT ON``.
The ``distinct()`` ``Queryset`` method now accepts an optional list of model
The ``distinct()`` ``QuerySet`` method now accepts an optional list of model
field names. If specified, then the ``DISTINCT`` statement is limited to these
fields. PostgreSQL is the only database backend shipped with Django that
supports this new functionality.
fields. This is only supported in PostgreSQL.
For more details, see the documentation for
:meth:`~django.db.models.query.QuerySet.distinct`.