触发器：
是一个特殊的存储过程；
通过增删改的动作来触发执行，没有参数，没有返回值；
create trigger Insert_Student --命名规范
on student --针对于哪一个表
for insert --针对于哪一个动作来触发
-- onclick = "show()"
as
触发执行的代码段
go
----------------------------------------------------
create trigger Delete_Info
on info
instead of delete
as
declare @c varchar(20)
select @c = code from deleted
delete from work where infocode=@c
delete from family where infocode=@c
delete from info where code=@c
go
create trigger Delete_Nation
on nation
for delete
as
go
1.for的意思是在动作执行之后触发
2.instead of delete 的意思是删除之前引发，可以理解为替代，写了这个之后，写的执行代码就没有用了，就被触发器的代码覆盖了
触发器常用的为级联删除：
create trigger delete_student
on student
instead of delete
as
--如果要删除student表数据，那么需要级联删除
declare @sno varchar(20);
set @sno = sno from deleted --deleted固定格式，为删除执行所能删除的数据，并没有执行删除，而是把他们显示出来，在这获得要删除的数据的sno，然后先删除其他表中此sno的数据
delete from score where sno = @sno;
delete from student where sno = @sno;
go