mysql 级联删除
create table a( id varchar(20) primary key, password varchar(20) not null)create table b( id int identity(1,1) primary key, name varchar(50) not null, userId varchar(20), foreign key (userId) references a(id) on delete cascade)表B创建了外码userId 对应A的主码ID，声明了级联删除
或者
alter table? b add constraint fk_b_userId foreign key (userId) references a(id) on delete cascade
测试数据：insert into a values ('11','aaa')insert into a values('23','aaa')insert into b values('da','11')insert into b values('das','11')insert into b values('ww','23')删除A表内id为‘11’的数据，发现B表内userId 为“11”也被数据库自动删除了，这就是级联删除delete from a where id='11'
?
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