MySql消息传递系统查询每次会话获取一条消息

Question

我有这3张桌子：

messages:
id  conv    snd text    created
1   1   1   asdasd      2012-12-15 23:10:53
3   1   1   zzz         2013-01-18 12:37:44
4   1   1   ssss        2013-01-18 12:42:01


msg_status
id  msg user    status
1   1   1       mine
2   1   4       unread
3   3   1       mine
4   3   4       unread
5   4   1       mine
6   4   4       unread


conv_users
conv    user
1       1
1       4
2       1
2       4
2       5

这是我站点中的一个消息传递系统，除了这个小查询之外，我已经准备就绪，可以使用对话中的最后一条消息来获取所有用户对话。

这意味着，如果用户A有2个会话，并且conv1中有10条消息，而conv2中只有2条消息，则他将获得两行：convId，messageId，messageText，messageDate等。

但是问题在于该消息应该是每次对话中的最后一条。

这是我已经完成的查询，但是没有用！

SELECT
  cu.conv,
  msg.snd,
  msg.text,
  mst.status,
  msg.created
FROM conv_users AS cu
  INNER JOIN messages AS msg
    ON cu.conv = msg.conv
  INNER JOIN msg_status AS mst
    ON msg.id = mst.msg
WHERE cu.user = 4
    AND mst.user = 4
ORDER BY msg.created DESC

如果我添加GROUP BY cu.conv它将显示第一条消息，而不是我想要的最后一条消息。

Answer 1

尝试将已查询的查询与max

SELECT
  cu.conv,
  msg.snd,
  msg.text,
  mst.status,
  msg.created
FROM conv_users AS cu
  INNER JOIN (SELECT MAX(id), conv FROM messages GROUP BY conv )messages AS msg ON cu.conv = msg.conv
  INNER JOIN (SELECT MAX(id) , msg FROM msg_status GROUP BY msg) AS mst ON msg.id = mst.msg 
WHERE cu.user = 4
    AND mst.user = 4
ORDER BY msg.created DESC