https://docs.djangoproject.com/en/2.2/ref/models/conditional-expressions/
# Case()接受任意数量的When()对象作为单独的参数。使用关键字参数提供其他选项。如果没有任何条件计算为TRUE，则default返回带有关键字参数的表达式。如果default未提供参数，None则使用。
# 示例
from datetime import date, timedelta
from django.db.models import Case, CharField, Value, When
>>> Client.objects.create(
    name='Jane Doe',
    account_type=Client.REGULAR,
    registered_on=date.today() - timedelta(days=36))
>>> Client.objects.create(
    name='James Smith',
    account_type=Client.GOLD,
    registered_on=date.today() - timedelta(days=5))
>>> Client.objects.create(
    name='Jack Black',
    account_type=Client.PLATINUM,
    registered_on=date.today() - timedelta(days=10 * 365))
>>> Client.objects.annotate(
	discount=Case(
    	When(account_type=Client.GOLD, then=Value('5%')),
        When(account_type=Client.PLATINUM, then=Value('10%')),
        default=Value('0%'),
        output_field=CharField(),
    ),
).value_list('name', 'discount')
<QuerySet [('Jane Doe', '0%'), ('James Smith', '5%'), ('Jack Black', '10%')]>