也许这不是最好的方法，但有时函数组\u concat（）可以是userfull，它将返回一个字符串，其中包含所有聚合值，按需要排序，并用逗号分隔（耦合值用空格分隔）。然后我使用函数split\u string（）来剪切字符串中的第一个id。

SELECT  
post_id, 
SPLIT_STRING( group_concat( forum_id, post_time ORDER BY post_time DESC ) ,' ',1 )as forum_id, 
SPLIT_STRING( group_concat( topic_id, post_time ORDER BY post_time DESC ) ,' ',1 )as topic_id ,
FROM posts 
GROUP BY topic_id 
ORDER BY post_time DESC
LIMIT 5

所以聚合的论坛id，发布时间如下：
81 2012-11-19 06:59:04,83 2012-11-19 16:07:46
因此，您需要使用整数和日期时间对的字符串表示，每个对用逗号分隔，因此我使用此函数来获取第一个int：

CREATE FUNCTION SPLIT_STRING(str VARCHAR(255), delim VARCHAR(12), pos INT)
RETURNS VARCHAR(255)
RETURN REPLACE(SUBSTRING(SUBSTRING_INDEX(str, delim, pos),
       LENGTH(SUBSTRING_INDEX(str, delim, pos-1)) + 1),
       delim, '');

注意：函数split\u string（str，delim，pos）在这里找到：相当于explode（）在mysql中处理字符串

它不是很漂亮，但很管用：

SELECT * FROM (SELECT  post_id, forum_id, topic_id FROM posts
ORDER BY post_time DESC) as temp
GROUP BY topic_id

如果选择group子句中未使用且不是聚合的属性，则结果未指定。i、 e您不知道从哪些行中选择其他属性(sql标准不允许这样的查询，但mysql更为宽松）。
然后应将查询写入，例如

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;

或者

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;

这对你也很有用。

SELECT *
FROM (
  SELECT post_id, forum_id, topic_id FROM posts
  ORDER BY post_time DESC
  LIMIT 5
) customeTable
GROUP BY topic_id

试试这样的

SELECT post_id, forum_id, topic_id 
FROM   (SELECT post_id, forum_id, topic_id
        FROM posts
        ORDER BY post_time DESC) 
GROUP BY topic_id 
ORDER BY topic_id desc
LIMIT 0,5

更改 order by 以及 limit 根据需要。