什么是触发器
触发器对表进行插入、更新、删除的时候会自动执行的特殊存储过程。触发器一般用在check约束更加复杂的约束上面。触发器和普通的存储过程的区别是：触发器是当对某一个表进行操作。诸如：update、insert、delete这些操作的时候，系统会自动调用执行该表上对应的触发器。SQL Server 2005中触发器可以分为两类：DML触发器和DDL触发器，其中DDL触发器它们会影响多种数据定义语言语句而激发，这些语句有create、alter、drop语句。
DML触发器分为：
1、 after触发器（之后触发）
a、 insert触发器
b、 update触发器
c、 delete触发器
2、 instead of 触发器 （之前触发）
其中after触发器要求只有执行某一操作insert、update、delete之后触发器才被触发，且只能定义在表上。而instead of触发器表示并不执行其定义的操作（insert、update、delete）而仅是执行触发器本身。既可以在表上定义instead of触发器，也可以在视图上定义。
触发器有两个特殊的表：插入表（instered表）和删除表（deleted表）。这两张是逻辑表也是虚表。有系统在内存中创建者两张表，不会存储在数据库中。而且两张表的都是只读的，只能读取数据而不能修改数据。这两张表的结果总是与被改触发器应用的表的结构相同。当触发器完成工作后，这两张表就会被删除。Inserted表的数据是插入或是修改后的数据，而deleted表的数据是更新前的或是删除的数据。
对表的操作
Inserted逻辑表
Deleted逻辑表
增加记录（insert）
存放增加的记录
无
删除记录（delete）
无
存放被删除的记录
修改记录（update）
存放更新后的记录
存放更新前的记录
Update数据的时候就是先删除表记录，然后增加一条记录。这样在inserted和deleted表就都有update后的数据记录了。注意的是：触发器本身就是一个事务，所以在触发器里面可以对修改数据进行一些特殊的检查。如果不满足可以利用事务回滚，撤销操作。
? 创建触发器
语法
create trigger tgr_name
on table_name
with encrypion –加密触发器
? for update...
as
? Transact-SQL
# 创建insert类型触发器
--创建insert插入类型触发器
if (object_id(‘tgr_classes_insert‘, ‘tr‘) is not null)
? drop trigger tgr_classes_insert
go
create trigger tgr_classes_insert
on classes
? for insert --插入触发
as
? --定义变量
? declare @id int, @name varchar(20), @temp int;
? --在inserted表中查询已经插入记录信息
? select @id = id, @name = name from inserted;
? set @name = @name + convert(varchar, @id);
? set @temp = @id / 2;?
? insert into student values(@name, 18 + @id, @temp, @id);
? print ‘添加学生成功！‘;
go
--插入数据
insert into classes values(‘5班‘, getDate());
--查询数据
select * from classes;
select * from student order by id;
insert触发器，会在inserted表中添加一条刚插入的记录。
# 创建delete类型触发器
--delete删除类型触发器
if (object_id(‘tgr_classes_delete‘, ‘TR‘) is not null)
? drop trigger tgr_classes_delete
go
create trigger tgr_classes_delete
on classes
? for delete --删除触发
as
? print ‘备份数据中……‘;?
? if (object_id(‘classesBackup‘, ‘U‘) is not null)
??? --存在classesBackup，直接插入数据
??? insert into classesBackup select name, createDate from deleted;
? else
??? --不存在classesBackup创建再插入
??? select * into classesBackup from deleted;
? print ‘备份数据成功！‘;
go
--
--不显示影响行数
--set nocount on;
delete classes where name = ‘5班‘;
--查询数据
select * from classes;
select * from classesBackup;
delete触发器会在删除数据的时候，将刚才删除的数据保存在deleted表中。
# 创建update类型触发器
--update更新类型触发器
if (object_id(‘tgr_classes_update‘, ‘TR‘) is not null)
? drop trigger tgr_classes_update
go
create trigger tgr_classes_update
on classes
? for update
as
? declare @oldName varchar(20), @newName varchar(20);
? --更新前的数据
? select @oldName = name from deleted;
? if (exists (select * from student where name like ‘%‘+ @oldName + ‘%‘))
??? begin
????? --更新后的数据
????? select @newName = name from inserted;
????? update student set name = replace(name, @oldName, @newName) where name like ‘%‘+ @oldName + ‘%‘;
????? print ‘级联修改数据成功！‘;
??? end
? else
??? print ‘无需修改student表！‘;
go
--查询数据
select * from student order by id;
select * from classes;
update classes set name = ‘五班‘ where name = ‘5班‘;
update触发器会在更新数据后，将更新前的数据保存在deleted表中，更新后的数据保存在inserted表中。
# update更新列级触发器
if (object_id(‘tgr_classes_update_column‘, ‘TR‘) is not null)
? drop trigger tgr_classes_update_column
go
create trigger tgr_classes_update_column
on classes
? for update
as
? --列级触发器：是否更新了班级创建时间
? if (update(createDate))
? begin
??? raisError(‘系统提示：班级创建时间不能修改！‘, 16, 11);
??? rollback tran;
? end
go
--测试
select * from student order by id;
select * from classes;
update classes set createDate = getDate() where id = 3;
update classes set name = ‘四班‘ where id = 7;
更新列级触发器可以用update是否判断更新列记录；
# instead of类型触发器
instead of触发器表示并不执行其定义的操作（insert、update、delete）而仅是执行触发器本身的内容。
创建语法
create trigger tgr_name
on table_name
with encryption
? instead of update...
as
? T-SQL
# 创建instead of触发器
if (object_id(‘tgr_classes_inteadOf‘, ‘TR‘) is not null)
? drop trigger tgr_classes_inteadOf
go
create trigger tgr_classes_inteadOf
on classes
? instead of delete/*, update, insert*/
as
? declare @id int, @name varchar(20);
? --查询被删除的信息，病赋值
? select @id = id, @name = name from deleted;
? print ‘id: ‘ + convert(varchar, @id) + ‘, name: ‘ + @name;
? --先删除student的信息
? delete student where cid = @id;
? --再删除classes的信息
? delete classes where id = @id;
? print ‘删除[ id: ‘ + convert(varchar, @id) + ‘, name: ‘ + @name + ‘ ] 的信息成功！‘;
go
--test
select * from student order by id;
select * from classes;
delete classes where id = 7;
# 显示自定义消息raiserror
if (object_id(‘tgr_message‘, ‘TR‘) is not null)
? drop trigger tgr_message
go
create trigger tgr_message
on student
? after insert, update
as raisError(‘tgr_message触发器被触发‘, 16, 10);
go
--test
insert into student values(‘lily‘, 22, 1, 7);
update student set sex = 0 where name = ‘lucy‘;
select * from student order by id;
# 修改触发器
alter trigger tgr_message
on student
after delete
as raisError(‘tgr_message触发器被触发‘, 16, 10);
go
--test
delete from student where name = ‘lucy‘;
# 启用、禁用触发器
--禁用触发器
disable trigger tgr_message on student;
--启用触发器
enable trigger tgr_message on student;
# 查询创建的触发器信息
--查询已存在的触发器
select * from sys.triggers;
select * from sys.objects where type = ‘TR‘;
--查看触发器触发事件
select te.* from sys.trigger_events te join sys.triggers t
on t.object_id = te.object_id
where t.parent_class = 0 and t.name = ‘tgr_valid_data‘;
--查看创建触发器语句
exec sp_helptext ‘tgr_message‘;
# 示例，验证插入数据
if ((object_id(‘tgr_valid_data‘, ‘TR‘) is not null))
? drop trigger tgr_valid_data
go
create trigger tgr_valid_data
on student
after insert
as
? declare @age int,
????? @name varchar(20);
? select @name = s.name, @age = s.age from inserted s;
? if (@age < 18)
? begin
??? raisError(‘插入新数据的age有问题‘, 16, 1);
??? rollback tran;
? end
go
--test
insert into student values(‘forest‘, 2, 0, 7);
insert into student values(‘forest‘, 22, 0, 7);
select * from student order by id;
# 示例，操作日志
if (object_id(‘log‘, ‘U‘) is not null)
drop table log
go
create table log(
id int identity(1, 1) primary key,
action varchar(20),
createDate datetime default getDate()
)
go
if (exists (select * from sys.objects where name = ‘tgr_student_log‘))
drop trigger tgr_student_log
go
create trigger tgr_student_log
on student
after insert, update, delete
as
if ((exists (select 1 from inserted)) and (exists (select 1 from deleted)))
begin
insert into log(action) values(‘updated‘);
end
else if (exists (select 1 from inserted) and not exists (select 1 from deleted))
begin
insert into log(action) values(‘inserted‘);
end
else if (not exists (select 1 from inserted) and exists (select 1 from deleted))
begin
insert into log(action) values(‘deleted‘);
end
go
--test
insert into student values(‘king‘, 22, 1, 7);
update student set sex = 0 where name = ‘king‘;
delete student where name = ‘king‘;
select * from log;
select * from student order by id;