SQL触发器实例(上)

 --1.) 创建测试用的表(testTable)
 if exists (select * from sysobjects where name='testTable')
 drop table testTable
 GO
 Create Table testTable
 (
 testField varchar(50)
 )

  select * from testTable

 --2.) 创建基于表(testTable)的触发器(testTrigger)
 IF EXISTS (Select name FROM sysobjects Where name = 'testTrigger' AND type = 'TR')
 Drop TRIGGER testTrigger
 GO
 Create Trigger testTrigger
 ON testTable
 for Insert,Delete,Update
 AS
 if exists(select * from inserted)
      if exists(select * from deleted)
          print '...更新'
      else
          print '...插入'
 else
      if exists(select * from deleted)
          print '...删除'
 Go

 --.) 操作testTable表,测试触发器testTrigger
 --分别执行Insert Into语句,Update语句,Delete语句,看看效果
 Insert Into testTable values ('testContent!')

 Update testTable Set testField = 'UpdateContent'

 Delete From testTable

 select * from testTable

 --用到的功能有:
         --1.如果我更改了学生的学号,我希望他的借书记录仍然与这个学生相关(也就是同时更改借书记录表的学号);
         --2.如果该学生已经毕业，我希望删除他的学号的同时，也删除它的借书记录，等等。

  --这时候可以用到触发器。对于需求1，创建一个Update触发器： 

      Create Trigger truStudent 

        On student      --在Student表中创建触发器
        instead of Update      --为什么事件触发
      As           --事件触发后所要做的事情
        if Update(stuID)
        begin 

          Update borrow
            Set stuID=i.stuID
            From borrow as br , Deleted as d ,Inserted as i      --Deleted和Inserted临时表
            Where br.stuID=d.stuID 

        end   

 drop trigger truStudent
 UPDATE student set stuID='' WHERE stuID=''

     Create trigger trdStudent
        On Student
        instead of Delete
      As
      Delete Borrow
 　　　From Borrow as br , Deleted as d Where br.StuID=d.stuID 

 drop trigger trdStudent
 delete FROM student WHERE stuID=''

 disable trigger trdStudent on Student

 select * from book
 select * from borrow
 select * from student

 --创建触发器（对删除表的约束）
 create trigger droptabel
 on database
 for drop_table
 as
 print'删除表吗？'
 print'不能删除表'
 rollback transaction
 go

 drop table students
 disable trigger droptabel on database　　　　--关闭触发器
 enable trigger droptabel on database

 CREATE TABLE students  --学生信息表
 (
   stuID CHAR(10) primary key,  --学生编号
   stuName  CHAR(10) NOT NULL ,     --学生名称
   major  CHAR(50) NOT NULL    --专业
 )
 GO

 CREATE TABLE borrowS  --借书表
 (
  borrowID  CHAR(10) primary key,    --借书编号
 stuID CHAR(10) NOT NULL, --学生编号
 BID  CHAR(10) NOT NULL,--图书编号
  T_time  datetime NOT NULL,   --借出日期
  B_time  datetime    --归还日期
 )
 GO

 SElect * from students
 select * from borrows

 Create Trigger truStudents 

        On students      --在Student表中创建触发器
        for Update      --为什么事件触发
      As           --事件触发后所要做的事情
        if Update(stuID)
        begin 

          Update borrows
            Set stuID=i.stuID
            From borrows as br , Deleted as d ,Inserted as i      --Deleted和Inserted临时表
            Where br.stuID=d.stuID 

        end   

 UPDATE studentS set stuID='' WHERE stuID=''

     Create trigger trStudent
        On Students
        for Delete
      As
        Delete borrows
          From borrows AS br , Deleted AS d
          Where br.stuID=d.stuID 

 drop trigger trStudent

 delete from students where stuID=''