MySQL支持外键的存储引擎只有InnoDB，在创建外键的时候，要求父表必须有对应的索引，子表在创建外键的时候也会自动创建对应的索引。在创建索引的时候，可以指定在删除、更新父表时，对子表进行的相应操作，包括RESTRICT、NOACTION、SET NULL和CASCADE。其中RESTRICT和NO ACTION相同，是指在子表有关联记录的情况下父表不能更新；CASCADE表示父表在更新或者删除时，更新或者删除子表对应记录；SET NULL则是表示父表在更新或者删除的时候，子表的对应字段被SET NULL。下面以一个新闻表说明，该新闻数据库的结构如下：

create database yynews;
use yynews;
#新闻类别表
create table categories(
catId int AUTO_INCREMENT primary key,
catName varchar(40) not null unique
)charset utf8;
#新闻表：
create table news(
newsId int AUTO_INCREMENT primary key,
title varchar(100) not null unique,
content text not null,
createTime timestamp not null,
catId int
)charset utf8;
#添加外键的引用
alter table news add constraint foreign key(catid) references categories(catid);
#评论表：
create table comments(
commId int AUTO_INCREMENT primary key,
content text not null,
createTime timestamp not null,
newsId int not null,
userIP char(15) not null
)charset utf8;
#添加外键的引用
alter table comments add constraint foreign key(newsid) references news(newsid);
#插入测试数据
insert into categories(catname) values("娱乐新闻");
insert into categories(catname) values("国际新闻");
insert into news(title,content,createTime,catId) values('test1','test1',now(),1);
insert into news(title,content,createTime,catId) values('test2','test2',now(),2);
insert into news(title,content,createTime,catId) values('test3','test3',now(),1);
insert into comments(content,createTime,newsId,userIP) values('you',now(),1,'127.0.0.1');
insert into comments(content,createTime,newsId,userIP) values('you',now(),2,'127.0.0.1');
insert into comments(content,createTime,newsId,userIP) values('you',now(),3,'127.0.0.1');
insert into comments(content,createTime,newsId,userIP) values('you',now(),1,'127.0.0.1');

mysql> select * from categories;
+-------+--------------+
| catId | catName |
+-------+--------------+
| 2 | 国际新闻 |
| 1 | 娱乐新闻 |
+-------+--------------+
2 rows in set (0.00 sec)

mysql> select * from news;
+--------+-------+---------+---------------------+-------+
| newsId | title | content | createTime | catId |
+--------+-------+---------+---------------------+-------+
| 1 | test1 | test1 | 2015-05-19 15:22:53 | 1 |
| 2 | test2 | test2 | 2015-05-19 15:22:53 | 2 |
| 3 | test3 | test3 | 2015-05-19 15:22:53 | 1 |
+--------+-------+---------+---------------------+-------+
3 rows in set (0.00 sec)

mysql> select * from comments;
+--------+---------+---------------------+--------+-----------+
| commId | content | createTime | newsId | userIP |
+--------+---------+---------------------+--------+-----------+
| 1 | you | 2015-05-19 15:22:53 | 1 | 127.0.0.1 |
| 2 | you | 2015-05-19 15:22:53 | 2 | 127.0.0.1 |
| 3 | you | 2015-05-19 15:22:53 | 3 | 127.0.0.1 |
| 4 | you | 2015-05-19 15:22:54 | 1 | 127.0.0.1 |
+--------+---------+---------------------+--------+-----------+
4 rows in set (0.00 sec)

mysql> delete from categories where catid=1;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`yynews`.
`comments`, CONSTRAINT `comments_ibfk_1` FOREIGN KEY (`newsId`) REFERENCES `news` (`newsId`))