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SELECT A.*,(SELECT MAX(LASTTIME) FROM B WHERE TITLE=A.TITLE),
(SELECT COUNT(1) FROM B WHERE TITLE=A.TITLE)
FROM TABLETITLE A
--1.
select a.*,b.回复时间
from a
left join b on a.帖子ID=b.帖子ID
and not exists(select 1 from b t where t.帖子ID=b.帖子ID and t.回复时间>b.回复时间)
--2.
select a.*,b.cnt as 回复总数
from a
left join
(select 帖子ID,count(1) as cnt from b group by 帖子ID) b
on a.帖子ID=b.帖子ID