外键约束

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')