什么是触发器
其是一种特殊的存储过程。一般的存储过程是通过存储过程名直接调用，而触发器主要是通过事件(增、删、改)进行触发而被执行的。其在表中数据发生变化时自动强制执行。
常见的触发器有两种：after(for)、instead of,用于insert、update、delete事件。
- after(for) 表示执行代码后，执行触发器
- instead of 表示执行代码前，用已经写好的触发器代替你的操作。
触发器语法：
create trigger 触发器的名字 on 操作表
for|after instead of
update|insert|delete
as
SQL语句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。
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。
触发器示例
Example1
–禁止用户插入数据(实际上是先插入，然后立刻将其删除！)
create trigger tr_insert on bank
for –for表示执行之后的操作
insert –即先执行了插入操作，同时在临时表中保存了插入记录
as
–执行完插入之后，在新生成的表中将刚刚插入的那条记录删除，
–而此时得到的刚刚插入的记录的id是通过临时表 inserted得到的
delete * from bank where cid=(select cid from inserted) 生成上面的触发器后，当用户再输入insert语句后就见不到效果了！
如：insert into bank values(‘0004’,10000),是插入不进数据库的。
Example2–删除谁就让谁的账户加上10元
create trigger tr_dalete on bank
instead of
delete
as
update bank balance=balance+10 where cid=(select cid from deleted)生成这个触发器之后，当用户输入delete语句后，对应的那个id不但没有被删除掉，而且他的账户增加了10元
如：delete from bank where cid=’0002’,执行完这句话后，编号为0002的账户会增加10元
细节讲解
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raiserrorif (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 = ‘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 = , @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;
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