MySQL选择不同用户的最新传入或传出消息

Question

我有一个表格结构，我会记录发送和接收的消息，如下所示;

member_id1是发件人，member_id2是接收者。

现在我要实现的是获取具有member_id为3的用户的最新消息的行。结果应该只包含2行，消息ID为2和4，因为2是member_id为3的用户与member_id为2的用户之间的最后一条消息，4是member_id为3的用户与member_id为1的用户之间的唯一消息。

结果不应包含message_id为3的行，因为它与member_id为3的用户无关，而message_id为1，因为message_id为2的消息已过期

希望我能帮助解决这个问题的人明白我的问题。

Answer 1

我想我已经为自己的问题找到了正确的答案。虽然我不太确定，但似乎效果很好。对于那些需要解决这个问题的人;尝试一下：

SELECT m1.* 
  FROM table_name m1 
  INNER JOIN (SELECT MAX(senddate) AS senddate, 
                     IF(member_id2 = 3, member_id1, member_id2 ) AS user 
                FROM table_name 
               WHERE (member_id1 = 3 AND delete1=0) OR 
                     (member_id2 = 3 AND delete2=0) 
              GROUP BY user) m2 
         ON m1.senddate = m2.senddate AND 
           (m1.member_id1 = m2.user OR m1.member_id2 = m2.user) 
     WHERE (member_id1 = 3 AND delete1=0) OR 
           (member_id2 = 3 AND delete2=0) 
   ORDER BY m1.senddate DESC

Answer 2

问题有点模糊，但据我所知，你在寻找这个？

 SELECT * FROM table_name WHERE member_id2 = 3 ORDER BY senddate DESC;

Answer 3

你可以使用UNION来组合它们

 (SELECT * FROM table_name WHERE member_id2 = 3  ORDER BY senddate DESC limit 1)
union 
 (SELECT * FROM table_name WHERE member_id1 = 3  ORDER BY senddate DESC limit 1)

或者如果您需要为每个成员获取最新发送/接收的内容，您可以加入查询，获取每个成员的最大发送日期：

select * from table_name t1 inner join 
(
 select member_id1, null as member_id2, max(senddate) as senddate 
 from table_name group by member_id1
 UNION
 select null as member_id1, member_id2, max(senddate) as senddate 
 from table_name group by member_id2
) r ON (t1.member_id1 = r.member_id1 OR t1.member_id2 = r.member_id2) 
AND t1.senddate = r.senddate