选择Last messages from last receiver。mysql

Question

我的桌子

id|receiver|sender|content|time
1 |   6    |  2   |  a    | 13:33
2 |   4    |  3   |  b    | 13:35
3 |   4    |  3   |  c    | 14:01
4 |   5    |  3   |  d    | 14:03
5 |   7    |  2   |  e    | 14:05
6 |   4    |  3   |  f    | 14:07

我的预期结果：

id|receiver|sender|content|time
6 |   4    |  3   |  f    | 14:07
3 |   4    |  3   |  c    | 14:01
2 |   4    |  3   |  b    | 13:35

目前我的方法是：

SELECT * FROM `meassages`
  WHERE `sender` = 3 AND `receiver` IN (
    SELECT `receiver` FROM `messages` ORDER BY `time` DESC LIMIT 1
  ) ORDER BY `time` DESC

有没有更简单的方法？

Answer 1

尽管您的查询看起来很好，但它假定最后一条消息来自sender = 3。因此，最好在子查询中包含一个where：

SELECT m.*
FROM messages m
WHERE m.sender = 3 AND 
      m.receiver = (SELECT m2.receiver
                    FROM messages m2
                    WHERE m2.sender = m.sender
                    ORDER BY m2.time DESC
                    LIMIT 1
                   )
ORDER BY m.time DESC;

使用=而不是IN强调了子查询返回一行。此外，我还添加了表别名和限定列名。

Answer 2

试着这样做：

SELECT m1.* 
FROM `meassages` AS m1
INNER JOIN
(
   SELECT receiver, MAX(time) AS LatestTime
   FROM messages 
   group by receiver
) AS m2  ON m1.receiver = m2.receiver 
        AND m1.`time` = m2.LatestTime
WHERE m1.`sender` = 3;

内部查询将为您提供每个接收器的最新时间，这应该是最后一个接收器。然后，使用基于此最新时间的再次连接表，您将删除除具有最新时间的行之外的所有行。