需要3张表，学生表student，选课表sc和课程表course，
select sname from student
where not exists(select * from course where not exists
(select * from sc where sno=student.sno and cno=course.cno))
SELECT St_Name
FROM St_Info
where St_ID in
(SELECT St_ID
FROM S_C_Info
WHERE C_No in
(SELECT C_No
FROM C_Info))
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