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ORM--分组查询,聚合查询,查询集QuerySet对象特性
2022-07-07 07:17:00 【chuntian_tester】
1.分组查询
1.1.使用 QuerySet对象.values(‘父表主键id’).annotate(聚合函数(‘从表模型类名小写')
1.2.会自动连接主从表,以外键字段作为分组条件
2.聚合运算
2.1.使用 QuerySet对象.aggregate(聚合函数('字段名')) 方法,返回字典数据
2.2.返回的字典数据中的key为 字段名__聚合函数名 小写
2.3.可以使用关键参数形式,返回的字典数据中的key为关键字参数名
qs = Projects.objects.filter(full_name__contains='x').aggregate(Count("id"))
qs = Projects.objects.filter(full_name__contains='x').aggregate(count=Count("id"))
3.查询集QuerySet对象的特性:
3.1 支持链式调用
可以在查询集上多次调用filter(),exclude()方法
3.2 惰性查询
仅仅在使用数据时才执行Sql语句,为了提升数据库的读写性能
3.3 会执行Sql语句场景:
len(),count(),通过索引取值,切片,for循环,print等
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