文章表：article_id, title, post_user, post_date
回复表：reply_id, article_id, reply_time, content
select
a.title, a.post_user, r.reply_time
from reply r
left join article a on a.article_id = r.article_id
where
r.reply_id =
(
select max(re.reply_id)
from reply re
where
re.article_id = r.article_id
