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MYSQL如何把两个结构相同的表组成一个表查询

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MYSQL如何把两个结构相同的表组成一个表查询
select * from
(select uid,count(*) as sc from `bbs_topics` group by uid) as a,
(select uid,count(*) as sc from `bbs_reply` group by uid) as b
where a.uid=b.uid
我向要取得两个表的数量总和并且group by uid
select uid,sum(sc) sl from
(select uid,count(*) as sc from `bbs_topics` group by uid
union
select uid,count(*) as sc from `bbs_reply` group by uid)
group by uid