MySQL离开加入以获取最后的消息和用户名

Question

我想获取当前用户与之对话的所有用户名和last_messages。

我有两张桌子：

发

用户

所以，如果当前用户的id是1，我应该获得bob和mike的行，但是我得到空的结果。你能帮我找错吗？

SELECT * FROM thread
LEFT JOIN user
ON ((thread.user1_id!=current_user AND thread.user1_id=user.id) 
    OR (thread.user2_id!=current_user AND thread.user2_id=user.id))
WHERE current_user = 1;

Answer 1

试试这个：

select
    calls.*
from User
left join (
    select
        User.Name, thread.user2_id as Partner_id 
    from User join thread on thread.user1_id = user.id
    union all
    select
        User.Name, thread.user1_id as Partner_id
    from User join thread on thread.user2_id = user.id
) calls
on calls.partner_id = User.id
where User.id = 1

注意从线程到User的双连接是如何完成的 - 一次将子查询中的调用两侧作为内连接，然后第二次作为外连接的外表。

Answer 2

请尝试这种方式

SELECT * FROM thread t
LEFT JOIN user u1
ON t.user1_id = u1.id
LEFT JOIN user u2
ON t.user2_id = u2.id
where t.user1_id = '1' or t.user2_id = '1'

应获取bob和mike的行（Thread line 1＆amp; 2）