MySQL 触发器(Triggers)

在MySQL中，触发器(trigger)是一段绑定于table上的sql程序(stored program)，这段程序在对表(db table)进行数据操作（insert, update, delete）时候触发并自动执行。

标准SQL定义了两种trigger类型：

• row level trigger
• statement level trigger

MySQL仅支持 row level trigger。

创建 Trigger

CREATE TRIGGER trigger_name
{BEFORE | AFTER} {INSERT | UPDATE| DELETE }
ON table_name FOR EACH ROW
BEGIN
trigger_body;
END

• tigger的名字需要在数据库级别唯一；

示例

use mysql_practice;

drop table if exists customer_audit;

create table customer_audit
(
	id int auto_increment primary key,
    customer_no varchar(50),
    first_name varchar(100),
    changed_date datetime,
    action varchar(50)
);


DROP TRIGGER IF EXISTS before_customer_update;

DELIMITER $$

CREATE TRIGGER before_customer_update
BEFORE UPDATE ON customer
FOR EACH ROW
BEGIN
	INSERT INTO customer_audit(customer_no, first_name, changed_date, action)
    values(OLD.no, OLD.first_name, now(), 'update');
    
    INSERT INTO customer_audit(customer_no, first_name, changed_date, action)
    values(NEW.no, NEW.first_name, now(), 'updated');
END $$

DELIMITER ;

调用更新customer表语sql句:

update customer set first_name = concat(first_name, '_u') where id = 3;

更新成功后，查看 customer_audit表数据，会插入两条新数据。

创建多triggers

MySQL 版本需要 5.7.2+

在MySQL 5.7.2之前，在db table对一个event只能创建一个trigger。

DELIMITER $$

CREATE TRIGGER trigger_name
{BEFORE|AFTER}{INSERT|UPDATE|DELETE} 
ON table_name FOR EACH ROW 
{FOLLOWS|PRECEDES} existing_trigger_name
BEGIN
    -- statements
END$$

DELIMITER ;

查看数据库的triggers

show triggers in mysql_practice;

show triggers from mysql_practice;

show triggers from mysql_practice like 'custom%'; -- like匹配表名

如果是多triggers，想查看action order，可以：

SELECT 
    trigger_name, 
    action_order
FROM
    information_schema.triggers
WHERE
    trigger_schema = 'mysql_practice'
ORDER BY 
    event_object_table , 
    action_timing , 
    event_manipulation;