初学数据库，有点不懂数据库查询问题exists

Question

select sname
from student
where not exists
(select *
from course
where not exists
(select *
from sc
where sno=student.sno
and cno=course.cno))
麻烦帮我解释一下每一个not exists的含义

Answer 1

加了not就代表相反，就是不存在的意思