MySQL触发器

0.准备工作

CREATE DATABASE dbtest17;

USE dbtest17;

1. 创建触发器

举例1：

① 创建数据表

CREATE TABLE test_trigger (
id INT PRIMARY KEY AUTO_INCREMENT,
t_note VARCHAR(30)
);

CREATE TABLE test_trigger_log (
id INT PRIMARY KEY AUTO_INCREMENT,
t_log VARCHAR(30)
);

② 查看表数据

SELECT * FROM test_trigger;

SELECT * FROM test_trigger_log;

③ 创建触发器

创建名称为before_insert_test_tri的触发器，向test_trigger数据表插入数据之前，

#向test_trigger_log数据表中插入before_insert的日志信息。

DELIMITER //

CREATE TRIGGER before_insert_test_tri
BEFORE INSERT ON test_trigger
FOR EACH ROW
BEGIN
INSERT INTO test_trigger_log(t_log)
VALUES(‘before insert…’);
END //

DELIMITER ;

④ 测试

INSERT INTO test_trigger(t_note)
VALUES(‘Tom…’);

SELECT * FROM test_trigger;

SELECT * FROM test_trigger_log;

举例2：

创建名称为after_insert_test_tri的触发器，向test_trigger数据表插入数据之后，

向test_trigger_log数据表中插入after_insert的日志信息。

DELIMITER $

CREATE TRIGGER after_insert_test_tri
AFTER INSERT ON test_trigger
FOR EACH ROW
BEGIN
INSERT INTO test_trigger_log(t_log)
VALUES(‘after insert…’);
END $

DELIMITER ;

测试

INSERT INTO test_trigger(t_note)
VALUES(‘Jerry2…’);

SELECT * FROM test_trigger;

SELECT * FROM test_trigger_log;

举例3：

定义触发器“salary_check_trigger”，基于员工表“employees”的INSERT事件，

在INSERT之前检查将要添加的新员工薪资是否大于他领导的薪资，如果大于领导薪资，

#则报sqlstate_value为’HY000’的错误，从而使得添加失败。

准备工作

CREATE TABLE employees
AS
SELECT * FROM atguigudb.employees;

CREATE TABLE departments
AS
SELECT * FROM atguigudb.departments;

DESC employees;

创建触发器

DELIMITER //

CREATE TRIGGER salary_check_trigger
BEFORE INSERT ON employees
FOR EACH ROW
BEGIN
#查询到要添加的数据的manager的薪资
DECLARE mgr_sal DOUBLE;

SELECT salary INTO mgr_sal FROM employees 
WHERE employee_id = NEW.manager_id;

IF NEW.salary > mgr_sal
	THEN SIGNAL SQLSTATE 'HY000' SET MESSAGE_TEXT = '薪资高于领导薪资错误';
END IF;
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END //

DELIMITER ;

测试

DESC employees;

添加成功：依然触发了触发器salary_check_trigger的执行

INSERT INTO employees(employee_id,last_name,email,hire_date,job_id,salary,manager_id)
VALUES(300,‘Tom’,‘tom@126.com’,CURDATE(),‘AD_VP’,8000,103);

添加失败

INSERT INTO employees(employee_id,last_name,email,hire_date,job_id,salary,manager_id)
VALUES(301,‘Tom1’,‘tom1@126.com’,CURDATE(),‘AD_VP’,10000,103);

SELECT * FROM employees;

2. 查看触发器

① 查看当前数据库的所有触发器的定义

SHOW TRIGGERS;

② 方式2：查看当前数据库中某个触发器的定义

SHOW CREATE TRIGGER salary_check_trigger;

③ 方式3：从系统库information_schema的TRIGGERS表中查询“salary_check_trigger”触发器的信息。

SELECT * FROM information_schema.TRIGGERS;

3. 删除触发器

DROP TRIGGER IF EXISTS after_insert_test_tri;