数据库 触发器基础用法（学生教师信息表）（备份删除行+删除多行：级联删除）

--触发器：触发器就是一种特殊的存储过程，只用来“增删改”
--触发器特殊的地方就在于，触发器是通过对数据库表的操作，来引发
--存储过程是通过人为exec来执行


select *from Student
create trigger Student_Insert --创建触发器
on Student --指定触发器所在的表，还可以写成after，for和after都是在操作后执行
for Insert --当执行insert操作的时候自动执行触发器
as
    update Student set sbirthday='1980-08-20'where Sno=110
go
insert into Student values(110,'王二小','男','1979-09-02',95033)








create trigger Student_Delete --创建触发器
on Student
instead of delete --替换某个操作，用触发器的过程来替换删除的操作，不在能够删除student表
as
    update Student set sbirthday='1980-08-20'where Sno=110
go
delete from Student





drop trigger Student_Delete --删除触发器






create trigger Student_Delete2  --删除某一条数据
on Student
instead of delete 
as
    delete from Score where Sno=105
    delete from Student where Sno=105
go
delete from Student
delete from Course where Cno='3-105'






create trigger Course_Delete --course表中的cno有主外键关系--deleted触发器中的临时表
on Course
instead of delete
as
    select *from deleted --deleted相当于虚拟的临时表，里面存着你要删除的那些数据
go    
delete from Course where Cno='3-105'





drop trigger Course_Delete --删除触发器








--一次性删除Course表数据，实用触发器替换删除操作，线删除外键表相关数据，再删除主键表course表相关数据

create trigger Course_Delete2--只能删除一行，要是删除多行由于赋值的问题遍历后只把最后一个数赋值执行，所以只删除了最后一行
on Course
instead of delete
as
        declare @cno varchar(20)
        select @cno = cno
        delete from score where cno=@cno
        delete from course where cno=@cno
go







create trigger Course_Delete4--一条一条删除多行
on Course
instead of delete
as
        delete from score where cno in (selete cno from deleted)
        delete from course where cno in (selete cno from deleted)
go








create trigger Course_Delete3
on Course
instead of delete
as
    declare @count int
    select @coount = count(*) from deleted
    if @count==1 --执行删除一行的操作
    begin
        declare @cno varchar(20)
        select @cno = cno
        delete from score where cno=@cno
        delete from course where cno=@cno
    end
    if @count>1 --循环遍历每一行，分别执行删除一行的操作
    begin
        delete from score where cno in (selete cno from deleted)
        delete from course where cno in (selete cno from deleted)
    end
go




drop table Scorebak
drop trigger Score_Delete
--删除前先备份数据到备份表之后再删除
create table Scorebak
(
    ids int identity(1,1) primary key,
    sno varchar(50),
    cno varchar(50),
    degree decimal(4,1),
    dayetime datetime
)
create trigger Score_Delete --备份删除
on Score
instead of delete
as
    declare @count int
    select @count = count(*) from deleted
    
    declare @i int
    set @i = 0
    while @i<@count --循环遍历deleted临时表的数据,然后转移到Scorebak
    begin
        declare @sno varchar(20)
        declare @cno varchar(20)
        declare @degree decimal(4,1)
        
        select top 1 @sno=sno,@cno=cno,@degree=degree from deleted 
        where sno not in (select top (@i) sno from deleted) or cno not in(select top (@i) cno from deleted) 
        --or degree not in (select top(@i) degree from score)--两条就能确定时，不用全写，这道题不用写degree not in，分页查询每一行
        
        insert into Scorebak values(@sno,@cno,@degree,getdate())
        
        delete from score where sno=@sno and cno=@cno
        
        set @i=@i+1        
    end
go
select *from Scorebak
select *from Score
delete from Score where Cno='3-245'