实例讲解MySQL数据库中应当如何建立外键

DROP TABLE IF EXISTS `biao`; 
CREATE TABLE `biao` ( 
id` int(11) NOT NULL auto_increment, 
title` varchar(11) default NULL, 
content` varchar(11) default NULL, 
PRIMARY KEY  (`id`) 
) ENGINE=InnoDB DEFAULT CHARSET=gb2312;

drop table cotton; 
create table cotton( 
 id       int primary key, 
 user     varchar(11), 
 email    varchar(11), 
 url      varchar(11), 
 content  varchar(11), 
 addTime  date, 
 biao_id  int, 
 constraint FK_biao_id foreign key (blog_id) references biao(id)  
)ENGINE=InnoDB DEFAULT CHARSET=gb2312;

DROP TABLE IF EXISTS `comment`; 
CREATE TABLE `comment` ( 
  `id` int(11) NOT NULL, 
  `user` varchar(11) default NULL, 
  `email` varchar(11) default NULL, 
  `url` varchar(11) default NULL, 
  `content` varchar(11) default NULL, 
  `addTime` date default NULL, 
  `blog_id` int(11) default NULL, 
  PRIMARY KEY  (`id`),                          
  KEY `FK_blog_id` (`blog_id`)                          
) ENGINE=InnoDB DEFAULT CHARSET=gb2312;                          
ALTER TABLE `comment` 
  ADD FOREIGN KEY (`blog_id`) REFERENCES `blog` (`id`);