SELECT Sname,Cno,Grade,平均成绩
FROM Student JOIN SC ON Student.Sno=SC.Sno
JOIN (SELECT Sno,AVG(Grade) 平均成绩 FROM SC GROUP BY Sno HAVING AVG(Grade)>=80) A ON Student.Sno=A.Sno
select Sname,Cno,Grade,(select avg(Grade)
from SC sc2 where sc2.Sno=sc1.Sno) as avgGrade
from Student,SC sc1
where sc1.Sno=Student.Sno and sc1.Sno in(select Sno
from SC sc3
group by Sno
having avg(Grade)>=80)
order by avgGrade desc;