MySQL：在INBOX中每位用户显示最后一条消息

Question

Here's my problem, in my inbox it shows multiple messages that I received from one user only. I need to to display my inbox just like facebook. Showing only one message per one user. Much like a conversation view. Wherein, the last message between me and a user is showed in the inbox (either his last message, or my last reply to him).

I have tried GROUP BY, but the results are not accurate. Some recent messages are not displayed and it is not sorted by date of last conversation.

Here's my database structure:

+-----------------------------------------------------------------------------------+
|                                  users_messages                                   |
+-----------------------------------------------------------------------------------+
| message_ID | sender |receiver| date | subject | body | unread | delete1 | delete2 |
+-----------------------------------------------------------------------------------+

+---------------------+
|        members      |
+---------------------+
| id | username | ... 
+----+----------+-----+

Here is my current query:

$result = $DB->query("SELECT p.*, p.sender as sender, m.*
FROM  " . DB_PREFIX . "messages p
LEFT JOIN " . DB_PREFIX . "members m ON p.sender=m.member_id
WHERE p.receiver='" . $SESSION->conf['member_id'] . "' AND delete2=0
GROUP BY p.sender ORDER BY p.senddate DESC
LIMIT " . (($page - 1) * $PREFS->conf['per_page']) . ", " . $PREFS->conf['per_page']);

Answer 1

SELECT t1.*, t2.*
FROM 
  users_messages AS t1
LEFT JOIN members AS t2
  ON t1.sender = t2.id
LEFT JOIN users_messages AS t3
  ON t1.sender = t3.sender AND t1.receiver = t3.receiver 
    AND t1.message_ID < t3.message_ID
WHERE (t1.sender = @userid OR t1.receiver = @userid) AND t3.message_ID IS NULL

I haven't tested so may have made an oversight but one option is you can do a user_messages self join on sender and reciver and t1.message_ID < t3.message_ID. Because the most recent message between a pair from t1 has no join candidates in t3 and it is a left join then the t3 columns will be null and you can select for this in the where clause to get the most recent.