php mysql Group By获取最新记录，而不是第一条记录

如果选择在group子句中未使用且不是聚合的属性，则结果不确定。 也就是说，
您不知道其他属性是从哪些行中选择的。（sql标准不允许此类查询，但MySQL更宽松）。

然后应将查询写为

SELECt post_id, forum_id, topic_idFROM posts pWHERe 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_idORDER BY post_time DESCLIMIT 5;

要么

SELECt post_id, forum_id, topic_id FROM postsNATURAL JOIN(SELECt forum_id, topic_id, max(post_time) AS post_time FROM posts GROUP BY forum_id, topic_id) pORDER BY post_timeLIMIT 5;