记录一下mysql触发器的一个小知识点

2021.11.30在做数据库关于触发器的实验的时候，突发奇想，如果触发器的触发动作体里含有外键的话，对触发器有什么影响：
通过下面的实验来学习一下：

• 触发器删除不含外键的表

DROP TABLE if EXISTS a;
DROP TABLE IF EXISTS b;

CREATE TABLE a(
    id INT PRIMARY KEY AUTO_INCREMENT,
    NAME VARCHAR(20)
); CREATE TABLE b(
    id INT PRIMARY KEY AUTO_INCREMENT,
    pwd VARCHAR(20)
); DELIMITER
    $;
CREATE TRIGGER TEST_A AFTER INSERT ON
    a FOR EACH ROW
BEGIN
    INSERT INTO b(pwd)
VALUES(NEW.NAME) ;
END $;
DELIMITER
    ;
DELIMITER
    $;
CREATE TRIGGER TEST_B AFTER DELETE
ON
    a FOR EACH ROW
BEGIN
    DELETE
FROM
    b
WHERE
    pwd = old.NAME ;
END $;
DELIMITER
    ;

测试样例

INSERT INTO a(NAME) 
values('hello1');
select * from a;
select * from b;
delete from a where NAME = 'hello';
select * from a;
select * from b;

• 给b添加外键

DROP TABLE IF EXISTS
    a;
DROP TABLE IF EXISTS
    b;
CREATE TABLE a(
    id INT PRIMARY KEY AUTO_INCREMENT,
    NAME VARCHAR(20)
); CREATE TABLE b(
    id INT PRIMARY KEY AUTO_INCREMENT,
    pwd VARCHAR(20),
    fk INT NOT NULL,
    FOREIGN KEY(fk) REFERENCES a(id)
); DELIMITER
    $;
CREATE TRIGGER TEST_A AFTER INSERT ON
    a FOR EACH ROW
BEGIN
    INSERT INTO b(pwd,fk)
VALUES(NEW.NAME, NEW.id) ;
END $;
DELIMITER
    ;
DELIMITER
    $;
CREATE TRIGGER TEST_B AFTER DELETE
ON
    a FOR EACH ROW
BEGIN
    DELETE
FROM
    b
WHERE
    pwd = old.NAME ;
END $;
DELIMITER
    ;
   
DELIMITER $;
CREATE TRIGGER TEST_C AFTER update
ON
    a FOR EACH ROW
BEGIN
    UPDATE b SET pwd = new.NAME WHERE
    pwd = old.NAME ;
END $;
DELIMITER
    ;

测试

INSERT INTO a(NAME) 
values('hello');
select * from a;
select * from b;
delete from a where NAME = 'hello1';
select * from a;
select * from b;
update a set NAME = 'new name' WHERE NAME = 'HELLO';
select * from a;
select * from b;

结果：有外键的删除就会出问题。

SQL 查询：


delete from a where NAME = 'hello1'
MySQL 返回： 文档

#1451 - Cannot delete or update a parent row: a foreign key constraint fails (`mydata`.`b`, CONSTRAINT `b_ibfk_1` FOREIGN KEY (`fk`) REFERENCES `a` (`id`))

总结

在触发器里面如果修改的表有外键的话，删除的时候就会出问题。没有外键则随便删除。增加，查询，更新都没有影响。