Mysql实现级联操作（级联更新、级联删除）

骑猪看日落 2023-03-13 11:46 3阅读 0赞

一、首先创建两张表stu，sc

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

create table stu(
    sid int UNSIGNED primary key auto_increment,
    name varchar(20) not null)
    TYPE=InnoDB charset=utf8;
    
    create table sc(
    scid int UNSIGNED primary key auto_increment,
    sid int UNSIGNED not null,
    score varchar(20) default '0',
    index (sid),   --外键必须加索引
    FOREIGN KEY (sid) REFERENCES stu(sid) ON DELETE CASCADE ON UPDATE CASCADE)
    TYPE=InnoDB charset=utf8;

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

![format_png][]

\--说明： 外键必须建立索引；

FOREIGN key(sid)  设置外键，把sid设为外键

REFERENCES stu(sid) 引用作用。引用stu表中的sid

ON DELETE CASCADE 级联删除  
ON UPDATE CASCADE 级联更新

二、向两张表插入数据

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

insert into stu (name) value ('zxf');
    insert into stu (name) value ('ls');
    insert into stu (name) value ('zs');
    insert into stu (name) value ('ww');
    
    insert into sc(sid,score) values ('1','98');
    insert into sc(sid,score) values ('1','98');
    insert into sc(sid,score) values ('2','34');
    insert into sc(sid,score) values ('2','98');
    insert into sc(sid,score) values ('2','98');
    insert into sc(sid,score) values ('3','56');
    insert into sc(sid,score) values ('4','78');
    insert into sc(sid,score) values ('4','98');

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

![format_png 1][]

注意：在sc表中插入数据时，若插入的sid为22，则会插入失败，违反外键约束，因为外键sid  
来自stu表中的id的主键，即stu中的id没有等于22的数据。

级联删除：将stu表中id为2的学生删除，该学生在sc表中的成绩也会级联删除

delete from stu where sid = '2';

![format_png 2][]

级联更新：stu表中id为3的学生更改为id为6，该学生在sc表中的对应id也会级联更新

update stu set sid=6 where sid='3';

![format_png 3][]

注意

删除表的时候必须先删除外键表(sc)，再删除主键表（stu）

![format_png 4][]

上图为违反外键约束，不能删除

![format_png 5][]

上图为正常删除，先删除sc表，再删除stu表！

**——现在的努力，只为小时候吹过的牛逼！ ——**

分类: [技术问题][Link 1]

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[Link 1]: https://www.cnblogs.com/zxf100/category/987761.html