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239 lines
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239 lines
8.9 KiB
Plaintext
=======================
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Conditional Expressions
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=======================
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.. currentmodule:: django.db.models.expressions
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Conditional expressions let you use :keyword:`if` ... :keyword:`elif` ...
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:keyword:`else` logic within filters, annotations, aggregations, and updates. A
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conditional expression evaluates a series of conditions for each row of a
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table and returns the matching result expression. Conditional expressions can
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also be combined and nested like other :doc:`expressions <expressions>`.
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The conditional expression classes
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==================================
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We'll be using the following model in the subsequent examples::
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from django.db import models
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class Client(models.Model):
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REGULAR = 'R'
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GOLD = 'G'
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PLATINUM = 'P'
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ACCOUNT_TYPE_CHOICES = (
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(REGULAR, 'Regular'),
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(GOLD, 'Gold'),
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(PLATINUM, 'Platinum'),
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)
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name = models.CharField(max_length=50)
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registered_on = models.DateField()
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account_type = models.CharField(
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max_length=1,
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choices=ACCOUNT_TYPE_CHOICES,
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default=REGULAR,
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)
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``When``
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--------
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.. class:: When(condition=None, then=None, **lookups)
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A ``When()`` object is used to encapsulate a condition and its result for use
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in the conditional expression. Using a ``When()`` object is similar to using
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the :meth:`~django.db.models.query.QuerySet.filter` method. The condition can
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be specified using :ref:`field lookups <field-lookups>` or
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:class:`~django.db.models.Q` objects. The result is provided using the ``then``
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keyword.
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Some examples::
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>>> from django.db.models import F, Q, When
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>>> # String arguments refer to fields; the following two examples are equivalent:
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>>> When(account_type=Client.GOLD, then='name')
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>>> When(account_type=Client.GOLD, then=F('name'))
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>>> # You can use field lookups in the condition
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>>> from datetime import date
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>>> When(registered_on__gt=date(2014, 1, 1),
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... registered_on__lt=date(2015, 1, 1),
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... then='account_type')
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>>> # Complex conditions can be created using Q objects
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>>> When(Q(name__startswith="John") | Q(name__startswith="Paul"),
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... then='name')
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Keep in mind that each of these values can be an expression.
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.. note::
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Since the ``then`` keyword argument is reserved for the result of the
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``When()``, there is a potential conflict if a
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:class:`~django.db.models.Model` has a field named ``then``. This can be
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resolved in two ways::
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>>> When(then__exact=0, then=1)
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>>> When(Q(then=0), then=1)
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``Case``
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--------
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.. class:: Case(*cases, **extra)
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A ``Case()`` expression is like the :keyword:`if` ... :keyword:`elif` ...
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:keyword:`else` statement in ``Python``. Each ``condition`` in the provided
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``When()`` objects is evaluated in order, until one evaluates to a
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truthful value. The ``result`` expression from the matching ``When()`` object
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is returned.
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A simple example::
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>>>
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>>> from datetime import date, timedelta
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>>> from django.db.models import Case, CharField, Value, When
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>>> Client.objects.create(
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... name='Jane Doe',
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... account_type=Client.REGULAR,
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... registered_on=date.today() - timedelta(days=36))
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>>> Client.objects.create(
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... name='James Smith',
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... account_type=Client.GOLD,
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... registered_on=date.today() - timedelta(days=5))
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>>> Client.objects.create(
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... name='Jack Black',
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... account_type=Client.PLATINUM,
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... registered_on=date.today() - timedelta(days=10 * 365))
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>>> # Get the discount for each Client based on the account type
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>>> Client.objects.annotate(
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... discount=Case(
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... When(account_type=Client.GOLD, then=Value('5%')),
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... When(account_type=Client.PLATINUM, then=Value('10%')),
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... default=Value('0%'),
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... output_field=CharField(),
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... ),
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... ).values_list('name', 'discount')
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<QuerySet [('Jane Doe', '0%'), ('James Smith', '5%'), ('Jack Black', '10%')]>
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``Case()`` accepts any number of ``When()`` objects as individual arguments.
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Other options are provided using keyword arguments. If none of the conditions
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evaluate to ``TRUE``, then the expression given with the ``default`` keyword
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argument is returned. If a ``default`` argument isn't provided, ``None`` is
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used.
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If we wanted to change our previous query to get the discount based on how long
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the ``Client`` has been with us, we could do so using lookups::
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>>> a_month_ago = date.today() - timedelta(days=30)
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>>> a_year_ago = date.today() - timedelta(days=365)
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>>> # Get the discount for each Client based on the registration date
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>>> Client.objects.annotate(
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... discount=Case(
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... When(registered_on__lte=a_year_ago, then=Value('10%')),
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... When(registered_on__lte=a_month_ago, then=Value('5%')),
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... default=Value('0%'),
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... output_field=CharField(),
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... )
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... ).values_list('name', 'discount')
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<QuerySet [('Jane Doe', '5%'), ('James Smith', '0%'), ('Jack Black', '10%')]>
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.. note::
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Remember that the conditions are evaluated in order, so in the above
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example we get the correct result even though the second condition matches
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both Jane Doe and Jack Black. This works just like an :keyword:`if` ...
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:keyword:`elif` ... :keyword:`else` statement in ``Python``.
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``Case()`` also works in a ``filter()`` clause. For example, to find gold
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clients that registered more than a month ago and platinum clients that
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registered more than a year ago::
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>>> a_month_ago = date.today() - timedelta(days=30)
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>>> a_year_ago = date.today() - timedelta(days=365)
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>>> Client.objects.filter(
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... registered_on__lte=Case(
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... When(account_type=Client.GOLD, then=a_month_ago),
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... When(account_type=Client.PLATINUM, then=a_year_ago),
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... ),
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... ).values_list('name', 'account_type')
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<QuerySet [('Jack Black', 'P')]>
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Advanced queries
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================
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Conditional expressions can be used in annotations, aggregations, lookups, and
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updates. They can also be combined and nested with other expressions. This
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allows you to make powerful conditional queries.
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Conditional update
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------------------
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Let's say we want to change the ``account_type`` for our clients to match
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their registration dates. We can do this using a conditional expression and the
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:meth:`~django.db.models.query.QuerySet.update` method::
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>>> a_month_ago = date.today() - timedelta(days=30)
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>>> a_year_ago = date.today() - timedelta(days=365)
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>>> # Update the account_type for each Client from the registration date
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>>> Client.objects.update(
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... account_type=Case(
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... When(registered_on__lte=a_year_ago,
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... then=Value(Client.PLATINUM)),
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... When(registered_on__lte=a_month_ago,
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... then=Value(Client.GOLD)),
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... default=Value(Client.REGULAR)
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... ),
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... )
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>>> Client.objects.values_list('name', 'account_type')
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<QuerySet [('Jane Doe', 'G'), ('James Smith', 'R'), ('Jack Black', 'P')]>
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.. _conditional-aggregation:
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Conditional aggregation
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-----------------------
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What if we want to find out how many clients there are for each
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``account_type``? We can use the ``filter`` argument of :ref:`aggregate
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functions <aggregation-functions>` to achieve this::
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>>> # Create some more Clients first so we can have something to count
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>>> Client.objects.create(
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... name='Jean Grey',
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... account_type=Client.REGULAR,
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... registered_on=date.today())
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>>> Client.objects.create(
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... name='James Bond',
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... account_type=Client.PLATINUM,
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... registered_on=date.today())
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>>> Client.objects.create(
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... name='Jane Porter',
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... account_type=Client.PLATINUM,
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... registered_on=date.today())
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>>> # Get counts for each value of account_type
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>>> from django.db.models import Count
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>>> Client.objects.aggregate(
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... regular=Count('pk', filter=Q(account_type=Client.REGULAR)),
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... gold=Count('pk', filter=Q(account_type=Client.GOLD)),
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... platinum=Count('pk', filter=Q(account_type=Client.PLATINUM)),
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... )
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{'regular': 2, 'gold': 1, 'platinum': 3}
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This aggregate produces a query with the SQL 2003 ``FILTER WHERE`` syntax
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on databases that support it:
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.. code-block:: sql
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SELECT count('id') FILTER (WHERE account_type=1) as regular,
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count('id') FILTER (WHERE account_type=2) as gold,
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count('id') FILTER (WHERE account_type=3) as platinum
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FROM clients;
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On other databases, this is emulated using a ``CASE`` statement:
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.. code-block:: sql
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SELECT count(CASE WHEN account_type=1 THEN id ELSE null) as regular,
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count(CASE WHEN account_type=2 THEN id ELSE null) as gold,
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count(CASE WHEN account_type=3 THEN id ELSE null) as platinum
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FROM clients;
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The two SQL statements are functionally equivalent but the more explicit
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``FILTER`` may perform better.
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