extra(额外,附加)  -嵌套查询
    """
        select
        id ,
        name ,
        (select count(*) from tb2) as n
    from tb1
    """
    v = models.UserInfo.objects.all().extra(
        select={
            'n':"select count(*) from app01_usertype where id>%s or id<%s", #第一个嵌套
            'm':"select count(*) from app01_usertype where id>%s or id<%s"  #第二个嵌套
        },

        select_params=[1,2,3,4]) #多个占位符按顺序填写
    for obj in v:
        print(obj.id,obj.name,obj.n)

    extra(额外,附加)  -where 条件
    v1 = models.UserInfo.objects.all().extra(
        where =["id=1","name='alex'"]
    )
    print(v1.query)

    v2 = models.UserInfo.objects.all().extra(
        where=["id=1 or id=%s", "name=%s"],
        params=[1,"alex"]
    )
    print(v2.query)

    extra(额外,附加)  -多表查询 ,没有外键的表关联  笛卡尔积
    v3 = models.UserInfo.objects.all().extra(
        tables=["app01_UserType"],
        where=["app01_UserInfo.ut_id = app01_UserType.id"]
    )
    print(v3.query)

    extra(额外,附加)  --全参查询样例
    v4 = models.UserInfo.objects.all().extra(
        select ={'newid':"select count(*) from app01_usertype where id>%s"},
        select_params=[1,],
        where=['age>%s',"app01_userinfo.ut_id=app01_usertype.id"],
        params=[18,],
        order_by=['-age'],
        tables=['app01_usertype']

    )
    print(v4.query)