Mysql-Cannot add foreign key constraint

首先创建学生表t_student

CREATE TABLE `t_student` (
  `id` int(12) unsigned NOT NULL AUTO_INCREMENT,
  `c_name` varchar(12) DEFAULT NULL,
  `c_class_id` int(12) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=11000 DEFAULT CHARSET=utf8

再创建学生扩展表t_student_extra

CREATE TABLE `t_student_extra` (
  `id` int(12) unsigned NOT NULL AUTO_INCREMENT,
  `c_student_id` int(12) DEFAULT NULL,
  `c_father_name` varchar(12) DEFAULT NULL,
  `c_mother_name` varchar(12) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `student_fk` (`c_student_id`)
) ENGINE=InnoDB AUTO_INCREMENT=11000 DEFAULT CHARSET=utf8

再创建t_student_extra的外键约束

Alter table `db_spring_demo`.`t_student_extra`  
  add foreign key (`c_student_id`) 
  references `db_spring_demo`.`t_student`(`id`) 
on update Restrict 
on delete Set null

此时抛出错误"Cannot add foreign key constraint"，错误码1215

其原因是主键为 UNSINGED类型的

修改数据库即可

ALTER TABLE `db_spring_demo`.`t_student`
  CHANGE `id` `id` INT (12) NOT NULL AUTO_INCREMENT;

ALTER TABLE `db_spring_demo`.`t_student_extra`
  CHANGE `id` `id` INT (12) NOT NULL AUTO_INCREMENT;