1.create view CS_student
AS select sno,sname,sage from Student where Sdept ='CS'

create view CS_student
AS select sno,sname,sage from Student where Sdept ='CS'** on check**

加了on check 后，对视图执行插入操作时，会检查插入的sdept是否为CS，若不是，则禁止插入
而没有加on check 时，对视图的插入中，sdept可以为null