php mysql Group By to get latest record, not first record

Question

The Table:

(`post_id`, `forum_id`, `topic_id`, `post_time`) 
(79, 8, 4, '2012-11-19 06:58:08');
(80, 3, 3, '2012-11-19 06:58:42'),
(81, 9, 9, '2012-11-19 06:59:04'),
(82, 11, 6, '2012-11-19 16:05:39'),
(83, 9, 9, '2012-11-19 16:07:46'),
(84, 9, 11, '2012-11-19 16:09:33'),

The Query:

SELECT  post_id, forum_id, topic_id FROM posts 
GROUP BY topic_id 
ORDER BY post_time DESC
LIMIT 5

The Results:

[0] => [post_id] => 84 [forum_id] => 9 [topic_id] => 11  
[1] => [post_id] => 82 [forum_id] => 11 [topic_id] => 6  
[2] => [post_id] => 81 [forum_id] => 9 [topic_id] => 9  
[3] => [post_id] => 80 [forum_id] => 3 [topic_id] => 3  
[4] => [post_id] => 79 [forum_id] => 8 [topic_id] => 4

The Problem:

How to rewrite the query so that it returns post_id -> 83 instead of post_id -> 81 ?

They both have the same forum and topic ids, but post_id -> 81 has an older date than post_id -> 83.

But it seems that Group By gets the 'first' record and not the 'newest' one.

I tried changing the query to

SELECT  post_id, forum_id, topic_id, MAX(post_time)

but that returns both post_id 81 and 83

Answer 1

If you select attributes that are not used in the group clause, and are not aggregates, the result is unspecified. I.e you don't know which rows the other attributes are selected from. (The sql standard does not allow such queries, but MySQL is more relaxed).

The query should then be written e.g. as

SELECT post_id, forum_id, topic_id
FROM posts p
WHERE post_time =
  (SELECT max(post_time) FROM posts p2
   WHERE p2.topic_id = p.topic_id
   AND p2.forum_id = p.forum_id)
GROUP BY forum_id, topic_id, post_id
ORDER BY post_time DESC
LIMIT 5;

or

SELECT post_id, forum_id, topic_id FROM posts
NATURAL JOIN
(SELECT forum_id, topic_id, max(post_time) AS post_time
 FROM posts
 GROUP BY forum_id, topic_id) p
ORDER BY post_time
LIMIT 5;