--刚学数据库，作业有道习题问『查询没有学全所有课的同学的学号、姓名』
select sno,sname from stu
except
select sno,sname from stu a
where not exists(select cno from sc b
where not exists(select sno,cno
from (select sno,cno from stu natural join sc)
where sno=a.sno and cno=b.cno)
)
--明明两个select语句都能成功查值，为何except会失败
--附表
stu:
sno
sname
sc:
sno
cno