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select s.sname from student s where s.sno not in (
select s1.sno from student s1 cross join course c1
where s1.sno+c1.cno not in
(select sc.sno+sc.cno from sc))
select sname from student where student.sno not in
(select distinct sc1.sno from
(select * from student s1 cross join course c1) sc1
left outer join sc on sc1.sno = sc.sno and sc1.cno = sc.cno
where sc.sno is null)