mysql - MySQL-为每个用户选择最后一行

这是我的查询：

 SELECT msgId as `ID`,msgFromUserId , msgToUserId ,
            DATE_FORMAT( msgDate,'%d/%m/%y %H:%i') as `time` ,  MID(msgText,1,30) as `text` ,
            (CASE WHEN (msgFromUserId=292646) then  b.user_login else  a.user_login END) as `sender`
            FROM tbl_messages inner join wp_users as a on tbl_messages.msgFromUserId=a.ID
            inner join wp_users as b on tbl_messages.msgToUserId=b.ID
            inner join tbl_forum_users u1 on tbl_messages.msgFromUserId=u1.user_ID
            inner join tbl_forum_users u2 on tbl_messages.msgToUserId=u2.user_ID
where (msgFromUserId=292646 or msgToUserId=292646)
and tbl_messages.msgId in (SELECT max(msgId) FROM tbl_messages   GROUP BY msgFromUserId, msgToUserId )
order by msgId desc

我得到这个：

我不想重复行。只是获得他们之间的最后对话

最佳答案

一种快速的解决方法是更改​​子查询

SELECT max(msgId) FROM tbl_messages   GROUP BY msgFromUserId, msgToUserId

至

SELECT max(msgId)
FROM tbl_messages
GROUP BY LEAST(msgFromUserId, msgToUserId), GREATEST(msgFromUserId, msgToUserId)

这会将来自292646到1以及从1到292646的消息组合在一起。

完整查询：

SELECT msgId as `ID`,msgFromUserId , msgToUserId ,
            DATE_FORMAT( msgDate,'%d/%m/%y %H:%i') as `time` ,  MID(msgText,1,30) as `text` ,
            (CASE WHEN (msgFromUserId=292646) then  b.user_login else  a.user_login END) as `sender`
            FROM tbl_messages inner join wp_users as a on tbl_messages.msgFromUserId=a.ID
            inner join wp_users as b on tbl_messages.msgToUserId=b.ID
            inner join tbl_forum_users u1 on tbl_messages.msgFromUserId=u1.user_ID
            inner join tbl_forum_users u2 on tbl_messages.msgToUserId=u2.user_ID
where (msgFromUserId=292646 or msgToUserId=292646)
and tbl_messages.msgId in (
    SELECT max(msgId)
    FROM tbl_messages
    GROUP BY LEAST(msgFromUserId, msgToUserId), GREATEST(msgFromUserId, msgToUserId)
)
order by msgId desc

为了提高性能，您还应该将user-id-condition移到子查询中：

SELECT max(msgId)
FROM tbl_messages
where (msgFromUserId=292646 or msgToUserId=292646) -- <-- here
GROUP BY LEAST(msgFromUserId, msgToUserId), GREATEST(msgFromUserId, msgToUserId)

要以最佳方式使用索引，应使用UNION ALL优化。但这看起来很复杂：

SELECT max(msgId)
FROM (
    SELECT msgToUserId as otherUserId, max(msgId) as msgId
    FROM tbl_messages
    WHERE msgFromUserId=292646
    GROUP BY msgToUserId

    UNION ALL

    SELECT msgFromUserId as otherUserId, max(msgId) as msgId
    FROM tbl_messages
    WHERE msgToUserId=292646
    GROUP BY msgFromUserId
) sub
GROUP BY otherUserId

请注意，这只是在WHERE子句（tbl_messages.msgId in (...)）中使用的子查询。

该子查询也可以用作派生表，因此我们可以使用tbl_messages将其联接：

SELECT msgId as `ID`,
       msgFromUserId,
       msgToUserId,
       DATE_FORMAT( msgDate,'%d/%m/%y %H:%i') as `time`,
       MID(msgText,1,30) as `text` ,
       (CASE WHEN (msgFromUserId=292646) then  b.user_login else  a.user_login END) as `sender`
FROM (
    SELECT max(msgId) as msgId
    FROM (
        SELECT msgToUserId as otherUserId, max(msgId) as msgId
        FROM tbl_messages
        WHERE msgFromUserId=292646
        GROUP BY msgToUserId
        UNION ALL
        SELECT msgFromUserId as otherUserId, max(msgId) as msgId
        FROM tbl_messages
        WHERE msgToUserId=292646
        GROUP BY msgFromUserId
    ) sub
    GROUP BY otherUserId
) sub
inner join tbl_messages on tbl_messages.msgId = sub.msgId
inner join wp_users as a on tbl_messages.msgFromUserId=a.ID
inner join wp_users as b on tbl_messages.msgToUserId=b.ID
inner join tbl_forum_users u1 on tbl_messages.msgFromUserId=u1.user_ID
inner join tbl_forum_users u2 on tbl_messages.msgToUserId=u2.user_ID
order by tbl_messages.msgId desc

您需要以下索引来支持子查询：

tbl_messages(msgFromUserId, msgToUserId [, msgId])
tbl_messages(msgToUserId, msgFromUserId [, msgId])