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select Sage,count(*)from S ,SC where S.sno=SC.sno group by Sage
select sage,cno , count(1) 课程人数 from
SC , S
where SC.Sno = S.Sno
group by Sage,cno
select sage,cno , count(1) 课程人数 from
SC , S
where SC.Sno = S.Sno
group by Sage,cno
select count(1) 课程人数 ,cno from
SC , S
where SC.Sno = S.Sno
group by Sage
select Sage,count(*)from S inner join SC
on S.sno=SC.sno
group by Sage
Select Sage,Count(*) as 人数
from S
left join SC
on S.Sno=SC.Sno
Group By Sage