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select m.*,
isnull((select count(1) from sc n where cno = m.cno) , 0 ) 选课人数
from course m
---修改
select
a.cname,isnull(count(b.sno),0) as 选课的人数
from
sc c
left join
student b
on
c.sno=b.sno
left join
course a
on
a.cno,=c.cno
select
a.cname,isnull(count(sno),0) as 选课的人数
from
sc c
left join
student b
on
c.sno=b.sno
left join
course a
on
a.cno,=c.cno,
数据呢??
--这样?
select cname,人数=isnull(cnt,0)
from course a left join
(select cno,cnt=count(*) from sc group by cno) b on a.cno=b.cno