MySQL定义外键的方法

MySQL定义外键的方法是每个学习MySQL的人都需要掌握的知识，下文就对MySQL定义外键的语句写法进行了详细的阐述，供您参考。

外键为MySQL带来了诸多的好处，下面就为您介绍MySQL定义外键的语句写法，以及MySQL定义外键过程中出现错误的处理方法，供您参考学习。

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。