爱华网CREATE TABLE categories (
category_id tinyint(3) unsigned NOT NULL AUTO_INCREMENT,
name varchar(30) NOT NULL,
PRIMARY KEY(category_id)
) ENGINE=INNODB;
INSERT INTO categories VALUES (1, ‘SQL Server’), (2, ‘Oracle’), (3, ‘PostgreSQL’), (4, ‘MySQL’), (5, ‘SQLite’);
CREATE TABLE members (
member_id INT(11) UNSIGNED NOT NULL,
name VARCHAR(20) NOT NULL,
PRIMARY KEY(member_id)
) ENGINE=INNODB;
CREATE TABLE articles (
article_id INT(11) unsigned NOT NULL AUTO_INCREMENT,
title varchar(255) NOT NULL,
category_id tinyint(3) unsigned NOT NULL,
member_id int(11) unsigned NOT NULL,
INDEX (category_id),
FOREIGN KEY (category_id) REFERENCES categories (category_id),CONSTRAINT fk_member FOREIGN KEY (member_id) REFERENCES members (member_id),
PRIMARY KEY(article_id)
) ENGINE=INNODB;
categories.category_id和articles.category_id、members.member_id和articles.member_id已经建立外键关系，只有articles.category_id的值存在与categories.category_id表中并且articles.member_id的值存在与members.member_id表中才会允许被插入或修改。：
删除外键定义 ：
定义外键的时候articles.member_id外键比articles.category_id子句多了一个CONSTRAINT fk_member。这个fk_member就是用来删除外键定义用的：
ALTER TABLE articles DROP FOREIGN KEY fk_member;
这样articles.member_id外键定义就被删除
如果定义时没有指定CONSTRAINT fk_symbol（即外键符号）时MySQL会自己创建一个，可以通过以下命令查看：
SHOW CREATE TABLE articles;
+———-+————————————+ | Table| Create Table| +———-+————————————+ | articles | CREATE TABLE `articles` ( `article_id` int(11) unsigned NOT NULL auto_increment, `category_id` tinyint(3) unsigned NOT NULL, `member_id` int(11) unsigned NOT NULL, `title` varchar(255) NOT NULL, PRIMARY KEY (`article_id`), KEY `category_id` (`category_id`), KEY `member_id` (`member_id`), CONSTRAINT `articles_ibfk_1` FOREIGN KEY (`category_id`) REFERENCES `categories` (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1| +———-+————————————+ 1 row in set (0.01 sec)
可以看出articles.category_id的外键符号为articles_ibfk_1，因此就可以执行以下命令删除外键定义：
ALTER TABLE articles DROP FOREIGN KEY articles_ibfk_1;
