Group by与Order by 一起怎么用？

问题场景：Mysql 按最新回复显示
回复表a
id,parentid,author,postdate帖子表b
id, title,content,author,createdate
要用mysql取得最新的回复人信息和帖子标题SELECT a . * , max( a.postdate ) AS maxpostdate, b.id, b.title
FROM a,b
WHERE a.parentid = b.id
GROUP BY a.parentid
ORDER BY maxpostdate DESC
LIMIT 0 , 10最终结果帖子列表没错但是回复人不是最新的而是最早发帖的麻烦帮忙看下哪里有问题谢谢先！