Javaweb-约束-外键约束

create table dept( id int PRIMARY key auto_increment, dep_name varchar(20), addr varchar(20) );
create table emp( id int PRIMARY key auto_increment, name varchar(20), age int, dep_id int );




insert into emp(name,age,dep_id) values('张三',20,1),('李四',20,1),('王五',20,1),('赵六',20,2),('孙七',22,2),('周八',18,2);
insert into dept(id,dep_name,addr) values(1,'研发部','广州'),(2,'销售部','深圳');





create table emp( id int auto_increment PRIMARY key, name varchar(20), age int, dep_id int, CONSTRAINT fk_emp_dept FOREIGN key(dep_id) REFERENCES dept(id) );
alter table emp drop FOREIGN key fk_emp_dept;
alter table emp add CONSTRAINT fk_emp_dept FOREIGN key(dep_id) REFERENCES dept(id);



添加外键约束后，删除研发部的记录发现失败了