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@@ -60,14 +60,16 @@ above:
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>>> Book.objects.filter(publisher__name="BaloneyPress").count()
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73
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- # Average price across all books.
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+ # Average price across all books, provide default to be returned instead
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+ # of None if no books exist.
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>>> from django.db.models import Avg
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- >>> Book.objects.aggregate(Avg("price"))
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+ >>> Book.objects.aggregate(Avg("price", default=0))
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{'price__avg': 34.35}
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- # Max price across all books.
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+ # Max price across all books, provide default to be returned instead of
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+ # None if no books exist.
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>>> from django.db.models import Max
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- >>> Book.objects.aggregate(Max("price"))
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+ >>> Book.objects.aggregate(Max("price", default=0))
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{'price__max': Decimal('81.20')}
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# Difference between the highest priced book and the average price of all books.
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@@ -632,3 +634,40 @@ aggregate that author count, referencing the annotation field:
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>>> from django.db.models import Avg, Count
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>>> Book.objects.annotate(num_authors=Count("authors")).aggregate(Avg("num_authors"))
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{'num_authors__avg': 1.66}
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+
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+Aggregating on empty querysets or groups
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+----------------------------------------
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+
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+When an aggregation is applied to an empty queryset or grouping, the result
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+defaults to its :ref:`default <aggregate-default>` parameter, typically
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+``None``. This behavior occurs because aggregate functions return ``NULL`` when
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+the executed query returns no rows.
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+
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+You can specify a return value by providing the :ref:`default
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+<aggregate-default>` argument for most aggregations. However, since
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+:class:`~django.db.models.Count` does not support the :ref:`default
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+<aggregate-default>` argument, it will always return ``0`` for empty querysets
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+or groups.
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+
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+For example, assuming that no book contains *web* in its name, calculating the
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+total price for this book set would return ``None`` since there are no matching
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+rows to compute the :class:`~django.db.models.Sum` aggregation on:
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+
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+.. code-block:: pycon
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+
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+ >>> from django.db.models import Sum
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+ >>> Book.objects.filter(name__contains="web").aggregate(Sum("price"))
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+ {"price__sum": None}
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+
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+However, the :ref:`default <aggregate-default>` argument can be set when
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+calling :class:`~django.db.models.Sum` to return a different default value if
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+no books can be found:
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+
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+.. code-block:: pycon
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+
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+ >>> Book.objects.filter(name__contains="web").aggregate(Sum("price", default=0))
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+ {"price__sum": Decimal("0")}
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+
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+Under the hood, the :ref:`default <aggregate-default>` argument is implemented
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+by wrapping the aggregate function with
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+:class:`~django.db.models.functions.Coalesce`.
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lufafajoshua