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| author | Adrian Holovaty <adrian@holovaty.com> | 2011-12-30 18:01:34 +0000 |
|---|---|---|
| committer | Adrian Holovaty <adrian@holovaty.com> | 2011-12-30 18:01:34 +0000 |
| commit | d3ef13044e73c64e7f8f46c5030040a5566012b3 (patch) | |
| tree | 798c9181bc542e895a3ff7d566c253a61fcf9d5d /docs/ref | |
| parent | afb21094a9fefd157f22b9706026625b66d10048 (diff) | |
Edited doc changes from [17244]
git-svn-id: http://code.djangoproject.com/svn/django/trunk@17302 bcc190cf-cafb-0310-a4f2-bffc1f526a37
Diffstat (limited to 'docs/ref')
| -rw-r--r-- | docs/ref/models/querysets.txt | 26 |
1 files changed, 16 insertions, 10 deletions
diff --git a/docs/ref/models/querysets.txt b/docs/ref/models/querysets.txt index b7bc647981..11e23c0250 100644 --- a/docs/ref/models/querysets.txt +++ b/docs/ref/models/querysets.txt @@ -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:: |