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触发器(trigger):监视某种情况,并触发某种操作,它是提供给程序员和数据分析员来保证数据完整性的一种方法,它是与表事件相关的特殊的存储过程,它的执行不是由程序调用,也不是手工启动,而是由事件来触发,例如当对一个表进行操作(insert,delete,update)时就会激活它执行。
触发器经常用于加强数据的完整性约束和业务规则等。触发器创建语法四要素:
1. 监视地点(table)
2. 监视事件(insert/update/delete)
3. 触发时间(after/before)
4. 触发事件(insert/update/delete)
其中:trigger_time 是触发器的触发事件,可以为 before(在检查约束前触发)或 after(在检查约束后触发);trigger_event 是触发器的触发事件,包括 insert、update 和 delete,可以使用 old 和 new 来引用触发器中发生变化的记录内容。
需要注意的:
1)需注意对同一个表的相同触发时间 (after/before) 的相同触发事件(insert/update/delete),只能定义一个触发器,否则报错
ERROR 1235 (42000): This version of MySQL doesn’t yet support ‘multiple triggers with the same action time and event for one table’
2)mysql 触发器中的 if 判断语法格式如下:(1). 循环中还可以有循环,(2).else 后面没有 then, (3).elseif!!!!不是 else if !!
if…then{
if…then{}
end if;
if…then{}
end if;
…
}
elseif…then..
else
end if ;
注意可以使用两个 if 循环,各自 end if 即可
3)注意 mysql 触发器中的 before 和 after 的区别:
before:(insert、update)可以对 new 进行修改,
after: 不能对 new 进行修改,两者都不能修改 old 数据。
对于 INSERT 语句, 只有 NEW 是合法的;
对于 DELETE 语句,只有 OLD 才合法;
对于 UPDATE 语句,NEW、OLD 可以同时使用。
after 是先完成数据的增删改,再触发,触发的语句晚于监视的增删改操作,无法影响前面的增删改动作;也就是说先插入订单记录,再更新商品的数量;
before 是先完成触发,再增删改,触发的语句先于监视的增删改,这样就可以对 new 进行修改了;
摘自网络的一个例子说明:
首先我们来创建两张表:
# 商品表
create table g
(
id int primary key auto_increment,
name varchar(20),
num int
);
# 订单表
create table o
(
oid int primary key auto_increment,
gid int,
much int
);
insert into g(name,num) values(‘ 商品 1 ’,10),(‘ 商品 2 ’,10),(‘ 商品 3 ’,10);
我们借助触发器来完成下订单之后,自动对商品表中相应商品做减法;如下:
create trigger tg2
after insert on o
for each row
begin
update g set num=num-new.much where id=new.gid;
end$
但是有个问题是,如果下订单数超过商品总数时,那么会导致商品表中产生负数,这样我们可以借助 before 来对订单中 new 值进行修改,保证商品表不会出现负数;
案例:当新增一条订单记录时,判断订单的商品数量,如果数量大于 10,就默认改为 10
DELIMITER $
create trigger tg6
beforeinsert on o
for each row
begin
if new.much > 10 then
set new.much = 10;
end if;
update g set num = num – new.much where id = new.gid;
end $
DELIMITER ;
4)不是说一个事务出发一次,如下这个事务修改了 10 行数,他会触发 10 次:
mysql> update blocks_infos set infos_id=1 where infos_id=2;
Query OK, 10 rows affected (0.22 sec)
Rows matched: 10 Changed: 10 Warnings: 0
5)针对 before 的情况,如果触发的操作没有成功,会导致原本的触发事件也不成功;
接下来记录下,我写的案例,当对一个表做增删改的时候,触发对另一表做相应的操作,
例如下面,如果 begin 后面有语法错误或者执行错误,那么会导致前面的 delete 失败;
DELIMITER $
create trigger tri_delete_blocks_infos1 before delete
on blocks_infos for each row
begin
DECLARE h int;
set h=(select intc from bidinfo.v_publish_info where id=old.infos_id);
if h is null then
update bidinfo.v_publish_info set intc=1 where id= old.infos_id;
else
update bidinfo.v_publish_info set intc=intc+1 where id= old.infos_id;
end if;
end $
DELIMITER ;
1. 关于 insert 的触发器:
我们的要求是当向 blocks_infos 的时候,先判断 blocks_infos_opensearch 表中有没有新 insert 的 infos_id,如果有就相应的 update, 没有的话就 insert,可以如下两种方法:
方法一使用 replace:
DELIMITER $
create trigger tri_insert_blocks_infos after insert
on blocks_infos for each row
begin
replace into blocks_infos_opensearch (infos_id,blocks) select infos_id,group_concat(blocks_id) blocks from blocks_infos where infos_id=new.infos_id group by infos_id;
end $
DELIMITER ;
注意关于 MySQL replace into 有三种形式(into 关键字可以省略):
1. replace into tbl_name(col_name, …) values(…)
2. replace into tbl_name(col_name, …) select …
3. replace into tbl_name set col_name=value, …
方法二:用 if 判断:
DELIMITER $
create trigger tri_insert_blocks_infos after insert
on blocks_infos for each row
begin
DECLARE c INT;
set c=(SELECT COUNT(infos_id) FROM blocks_infos WHERE infos_id=new.infos_id);
if c=1 then
insert into blocks_infos_opensearch select infos_id,GROUP_CONCAT(blocks_id) blocks FROM blocks_infos WHERE infos_id=new.infos_id;
elseif c>1 then
UPDATE blocks_infos_opensearch SET blocks= (SELECT GROUP_CONCAT(blocks_id) blocks FROM blocks_infos WHERE infos_id=new.infos_id ) WHERE infos_id= new.infos_id;
end if ;
end $
DELIMITER ;
2. 关于 delete 的触发器:
DELIMITER $
CREATE TRIGGER tri_delete_blocks_infos after DELETE
ON blocks_infos FOR EACH ROW
BEGIN
DECLARE c INT;
SET c=(SELECT COUNT(infos_id) FROM blocks_infos WHERE infos_id=old.infos_id);
IF c=0 THEN
DELETE FROM blocks_infos_opensearch WHERE infos_id=old.infos_id;
ELSEIF c>0 THEN
UPDATE blocks_infos_opensearch SET blocks= (SELECT GROUP_CONCAT(blocks_id) blocks FROM blocks_infos WHERE infos_id=old.infos_id ) WHERE infos_id= old.infos_id;
END IF;
END $
DELIMITER ;
3. 关于 update 的触发器:
DELIMITER $
CREATE TRIGGER tri_update_blocks_infos after update
ON blocks_infos FOR EACH ROW
BEGIN
DECLARE c INT;
DECLARE d varchar(1000);
DECLARE h varchar(1000);
SET c=(SELECT COUNT(infos_id) FROM blocks_infos WHERE infos_id=old.infos_id);
set d=(SELECT GROUP_CONCAT(blocks_id) blocks FROM blocks_infos WHERE infos_id=old.infos_id);
set h=(SELECT GROUP_CONCAT(blocks_id) blocks FROM blocks_infos WHERE infos_id=new.infos_id);
IF c=0 THEN
DELETE FROM blocks_infos_opensearch WHERE infos_id=old.infos_id;
ELSEIF c>0 THEN
UPDATE blocks_infos_opensearch SET blocks= d WHERE infos_id= old.infos_id;
UPDATE blocks_infos_opensearch SET blocks= h WHERE infos_id= new.infos_id;
END IF;
END $
DELIMITER ;
另一个需求是需要当对表 blocks_infos 做相关处理的时候,会触发另一个表 bidinfo.v_publish_info 做相应的处理,因为前面已经建立了 after insert on blocks_infos,不能再建立 after insert
on blocks_infos, 所以只能创建 before insert on blocks_infos,如下创建了三个:
1)insert
DELIMITER $
create trigger tri_insert_blocks_infos1 before insert
on blocks_infos for each row
begin
DECLARE d int;
set d=(select intc from bidinfo.v_publish_info where id=new.infos_id);
if d is null then
update bidinfo.v_publish_info set intc=1 where id= new.infos_id;
else
update bidinfo.v_publish_info set intc=intc+1 where id= new.infos_id;
end if;
end $
DELIMITER ;
2)delete
DELIMITER $
create trigger tri_delete_blocks_infos1 before delete
on blocks_infos for each row
begin
DECLARE h int;
set h=(select intc from bidinfo.v_publish_info where id=old.infos_id);
if h is null then
update bidinfo.v_publish_info set intc=1 where id= old.infos_id;
else
update bidinfo.v_publish_info set intc=intc+1 where id= old.infos_id;
end if;
end $
DELIMITER ;
3)update,注意可以只用两个 if 循环!
DELIMITER $
create trigger tri_update_blocks_infos1 before update
on blocks_infos for each row
begin
DECLARE j int;
DECLARE i int;
set i=(select intc from bidinfo.v_publish_info where id=new.infos_id);
set j=(select intc from bidinfo.v_publish_info where id=old.infos_id);
if j is null then
update bidinfo.v_publish_info set intc=1 where id= old.infos_id;
else
update bidinfo.v_publish_info set intc=intc+1 where id= old.infos_id;
end if;
if i is null then
update bidinfo.v_publish_info set intc=1 where id= new.infos_id;
else
update bidinfo.v_publish_info set intc=intc+1 where id= new.infos_id;
end if;
end $
DELIMITER ;
小结:触发器中的 new 和 old, 可以理解为处理过的整行数据,可以通过 new. 字段名来取出那个字段的值,并且 alter 和 before 都不能修改 old 的值,但是 before 可以修改 new 的值,还需要注意对同一个表的相同触发时间 (after/before) 的相同触发事件(insert/update/delete),只能定义一个触发器,并且 before 的形式的触发器需要保证触发端和被触发端都得成功才能成功!