解释一下这个语句

Question

现有三张表 student(sno,sname,sdepartment),课程表course(Cno,Cname)
         选课表(sno,cno,grade)(注:学号,课程号,成绩)
现在要查询选修了全部课程的学生姓名,SQL的语句为
SELECT Sname
FROM Student
WHERE NOT EXISTS
                  （SELECT *
                        FROM Course
                        WHERE NOT EXISTS
                                      (SELECT *
                                       FROM SC
                                       WHERE Sno= Student.Sno
                                             AND Cno= Course.Cno
                                       ）
                       ）；
哪位高手给解释一下该语句的执行过程?谢谢

Answer 1

查询表 Student中的Cno不在SC 表中和Course表中的数据

Answer 2

查找sc.Sno<> Student.Sno and sc.Cno<> Course.Cno的记录

Answer 3

先執行SELECT *
                                       FROM SC
                                       WHERE Sno= Student.Sno
                                             AND Cno= Course.Cno
再執行SELECT *
                        FROM Course
                        WHERE NOT EXISTS
最後執行SELECT Sname
FROM Student
WHERE NOT EXISTS