查询选修了全部课程的学生姓名
：student 是学生表 course 是选课表 sc 是课程表
select sname
from student
where not exists
（select *
from course
where not exists
(select *
from sc
where sno =student.sno
and cno=course.cno)
：student 是学生表 course 是选课表 sc 是课程表
select sname
from student
where not exists
（select *
from course
where not exists
(select *
from sc
where sno =student.sno
and cno=course.cno)
