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主题： MySQL定义外键的方法

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发表于：

2015-5-27 11:09:57 | [全部帖] [楼主帖]

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mysql> CREATE TABLE categories (
-> category_id tinyint(3) unsigned NOT NULL AUTO_INCREMENT,
-> name varchar(30) NOT NULL,
-> PRIMARY KEY(category_id)
-> ) ENGINE=INNODB;
Query OK, 0 rows affected (0.36 sec)
mysql> INSERT INTO categories VALUES (1, ‘SQL Server’), (2, ‘Oracle’), (3, ‘PostgreSQL’), (4, ‘MySQL’), (5, ‘SQLite’);
Query OK, 5 rows affected (0.48 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql> CREATE TABLE members (
-> member_id INT(11) UNSIGNED NOT NULL,
-> name VARCHAR(20) NOT NULL,
-> PRIMARY KEY(member_id)
-> ) ENGINE=INNODB;
Query OK, 0 rows affected (0.55 sec)
mysql> INSERT INTO members VALUES (1, ‘test’), (2, ‘admin’);
Query OK, 2 rows affected (0.44 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> 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;
Query OK, 0 rows affected (0.63 sec)

注意：对于非InnoDB表，FOREIGN KEY子句会被忽略掉。
如果遇到如下错误：

ERROR 1005: Can’t create table ‘./test/articles.frm’ (errno: 150)

请仔细检查以下定义语句，常见的错误一般都是表类型不是INNODB、相关联的字段写错了、缺少索引等等。

至此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。例如：

mysql> INSERT INTO articles (category_id, member_id, title) VALUES (6, 1, ‘foo’);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test/articles`, CONSTRAINT `articles_ibfk_1` FOREIGN KEY (`category_id`)REFERENCES `categories` (`id`))
mysql> INSERT INTO articles (category_id, member_id, title) VALUES (3, 3, ‘foo’);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test/articles`, CONSTRAINT `fk_member` FOREIGN KEY (`member_id`) REFERENCES `members` (`member_id`))

可见上面两条语句都会出现错误，因为在categories表中并没有category_id=6、members表中也没有member_id=3的记录，所以不能插入。而下面这条SQL语句就可以。

mysql> INSERT INTO articles (category_id, member_id, title) VALUES (3, 2, ‘bar’);
Query OK, 1 row affected (0.03 sec)

以上就是MySQL定义外键的方法介绍。

--转自