MySQL的 - 选择最后一排为每个用户

这是我的查询：MySQL的 - 选择最后一排为每个用户

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。只是让他们之间的谈话

来源

2017-03-05 Adi

你能做的就是'订单DESC'你从'last到first'的顺序获得行，然后'LIMIT 1'，你得到唯一的最后一行，即。第一行 – Alen

您是否对表和列定义有严重问题？记录107和26具有相同的发送者。当有其他'FromUserId'时它不应该改变吗？ – Whencesoever

你为什么加入'tbl_forum_users'？你在哪里使用'u1'和'u2'？ –

快速修复可能会改变你的子查询

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

到

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

这将组消息从292646到1和从1到292646的最后一行一起。

完整的查询：

为了提高性能，你也应该将用户ID条件为子查询：

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

要使用索引，你应该使用的最佳方法UNION ALL优化。但是，这会看起来非常复杂：

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

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

此子查询也可以用来作为派生表，所以我们可以tbl_messages加入它：

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

您需要以下指标来支持子查询：

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

来源

2017-03-05 12:30:12

MySQL的 - 选择最后一排为每个用户

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