MySQL-为每个用户选择最后一行

Question

this is my query :

 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

i get this :

I dont want duplication row. Just get the last line of conversation between them

Answer 1

A fast fix could be to change your subquery

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

to

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

This will group messages from 292646 to 1 and from 1 to 292646 together.

Complete query:

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

To improve the performance you should also move the user-id-condition into the subquery:

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

To use the indexes the best way you should use a UNION ALL optimisation. But this is going to look pretty complex:

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

Note this is only the subquery to use in the WHERE clause ( tbl_messages.msgId in (...) ).

This subquery can also be used as a derived table, so we can join it with tbl_messages :

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

You need the following indexes to support the subquery:

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