你都分類了。只能求同類中的reply_date了
SELECT a.posts_id,a.reply_date,a.send_time
FROM
TABLE A,
(
SELECT posts_id,MAX(reply_date) as reply_date
FROM TABLE
GROUP BY posts_id
) B
ON A.posts_id=B.posts_id AND A.reply_date=B.reply_date
ORDER BY B.send_time DESC

group by posts_id
order by reply_date desc,send_time desc
應該可以了