MySQL：选择每对的最后一行

Question

I have a little problem to solve, but I can't. I have the following chat table:

id      user    friend      msg                     date
----------------------------------------------------------------------
1       1       2           Hello Bob!              2014-07-04 01:00
2       1       2           How are you doing?      2014-07-04 01:01
3       2       1           I'm fine bro!           2014-07-04 02:30
4       1       3           Hey Mark :D             2014-07-04 02:31
5       3       1           Yo!                     2014-07-04 02:32
6       4       1           Wassup?!                2014-07-04 07:00

I'm working on a PHP getInbox($uid) method that returns an array of the last message of each pair (user and friend). I tried a SELECT query with GROUP BY friend , but it is incomplete.

SELECT * FROM `chat`
GROUP BY `friend` 
WHERE `user` = $uid OR `friend` = $uid

The desired result is:

id      user    friend      msg                     date
----------------------------------------------------------------------
3       2       1           I'm fine bro!           2014-07-04 02:30
5       3       1           Yo!                     2014-07-04 02:32
6       4       1           Wassup?!                2014-07-04 07:00

I would appreciate a help!

Answer 1

SELECT c.* FROM
chat c
JOIN
(SELECT
    max(id) max_id,
    (CASE WHEN user < friend THEN user ELSE friend END) user_a,
    (CASE WHEN user < friend THEN friend ELSE user END) user_b
FROM chat
    GROUP BY user_a, user_b) t1 ON t1.max_id = c.id

The case statements select (user,friend) in ascending order. For example, both (1,2) and (2,1) will be converted to (1,2). An ordered pair uniquely identifies a conversation. Finally, the latest id is selected for each ordered pair and rows having those ids are displayed from the chat table.