SQL查询：连接+条件+顺序

Question

我有3张桌子：
user（use_id，use_role），
消息（mes_id，from_use_id [fk-＆gt; user.use_id]，mes_create_at），
message_user（mes_use_id，mes_id [fk-＆gt; message.mes_id]，to_use_id [fk-＆gt; user.use_id]）

用户可以是use_role = 1（学生）或use_role = 2（老师） 我必须让所有use_type = 1（学生）的用户按照与教师的最后联系进行排序。无论学生写信给老师或老师写信给学生。

Answer 1

我假设您使用ms sql：

select *
from
(
    select student.use_id, Message.mes_create_at, Message.from_use_id, Message_User.to_use_id
    from User student
    join Message on Message.use_id = student.use_id and student.use_role = 1
    join Message_User on Message_User.mes_id = Message.mes_id
    where Message_User.to_use_id in (select use_id from User where use_role = 2)

    union

    select Message_User.to_use_id, Message.mes_create_at, Message.from_use_id, Message_User.to_use_id
    from User teacher
    join Message on Message.use_id = teacher.use_id and teacher.use_role = 2
    join Message_User on Message_User.mes_id = Message.mes_id
    where Message_User.to_use_id in (select use_id from User where use_role = 1)
) as temp
order by mes_create_at desc