外键约束

Posted 2023-02-28 wangjiuwang

1.创建class，课程表
DROP TABLE IF EXISTS `class`;
CREATE TABLE `class` (
  `scno` varchar(10) NOT NULL,
  `scname` varchar(10) default NULL,
  PRIMARY KEY  (`scno`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
2.创建student，学生表
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
  `sid` varchar(10) NOT NULL,
  `sname` varchar(10) default NULL,
  `scno` varchar(10) default NULL,
  PRIMARY KEY  (`sid`),
  KEY `s_sc` (`scno`),
  CONSTRAINT `s_sc` FOREIGN KEY (`scno`) REFERENCES `class` (`scno`) ON DELETE NO ACTION ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


3.实例
//插入时外键不存在于外键表中时插入不成功insert into student(sid,sname,scno)values('2','222','10')
//没有外键可以插入insert into student(sid,sname)values('1','111');
//课程表里有数据才可以insert into class(scno,scname)values('10','aaa');insert into student(sid,sname,scno)values('2','222','10')