SELECT snum,sname
FROM student
WHERE NOT EXISTS(
                SELECT *
                FROM course
                WHERE NOT EXISTS(
                                SELECT *
                                FROM sc
                                WHERE student.snum=sc.snum
                                AND )
                )
        

这个比较麻烦，因为所有选修课时多少个，是个问题，给你一个建议，用count（*）统计学生选修课表，最多的那个应该就是选修全部的了

应该与course表无关!
SELECT snum,sname
FROM student
WHERE NOT EXISTS(
                SELECT *
                FROM sections
                WHERE NOT EXISTS(
                                SELECT *
                                FROM sc
                                WHERE student.snum=sc.snum
                                AND sc.secnum=sections.secnum)
                )