拿《数据库系统概论》的例子说：

sleect Sname
from Student
where not exists(select *
                 from Course
                 where not exists (select *
                                   from sc
                                   where Sno=Student.Sno and
                                         Cno=))

请问：
1.执行过程是什么呢？
2.如何来理解它呢？我总是理解不了.....
3.用其他方法可以吗？