php mysql组获得最新记录,不是第一次记录

桌子：

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

查询：

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

结果：

[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

问题：

如何重写查询以使其返回post_id – > 83而不是post_id – > 81？

他们都有相同的论坛和主题标识,但post_id – > 81的日期大于post_id – > 83.

但是看来,Group By获得了“第一”的记录,而不是“最新的”记录.

我尝试将查询更改为

SELECT  post_id,topic_id,MAX(post_time)

但是这会返回post_id 81和83

然后应该将查询写入如

SELECT post_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,post_id
ORDER BY post_time DESC
LIMIT 5;

要么

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

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