第15章_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...');

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('Jerry...');

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;

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;
			
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. 查看当前数据库中某个触发器的定义
SHOW CREATE TRIGGER salary_check_trigger;

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;