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[4.2.x] Refs #34808 -- Doc'd that aggregation functions on empty groups can return None.

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Backport of 78b5c9075348aa12da2e024f6ece29d1d652dfdd from main
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David Sanders 2023-09-20 23:58:23 +10:00 committed by Natalia
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docs/ref/models/querysets.txt
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@ -3925,14 +3925,15 @@ documentation to learn how to create your aggregates.
currently emulates these features using a text field. Attempts to use
aggregation on date/time fields in SQLite will raise ``NotSupportedError``.
.. admonition:: Empty queryset
.. admonition:: Empty querysets or groups
Aggregation functions return ``None`` when used with an empty
``QuerySet``. For example, the ``Sum`` aggregation function returns ``None``
instead of ``0`` if the ``QuerySet`` contains no entries. To return another
value instead, pass a value to the ``default`` argument. An exception is
``Count``, which does return ``0`` if the ``QuerySet`` is empty. ``Count``
does not support the ``default`` argument.
Aggregation functions return ``None`` when used with an empty ``QuerySet``
or group. For example, the ``Sum`` aggregation function returns ``None``
instead of ``0`` if the ``QuerySet`` contains no entries or for any empty
group in a non-empty ``QuerySet``. To return another value instead, define
the ``default`` argument. ``Count`` is an exception to this behavior; it
returns ``0`` if the ``QuerySet`` is empty since ``Count`` does not support
the ``default`` argument.
All aggregates have the following parameters in common: