mysql中怎么实现外键关联

mysql中怎么实现外键关联，针对这个问题，这篇文章详细介绍了相对应的分析和解答，希望可以帮助更多想解决这个问题的小伙伴找到更简单易行的方法。

> show create table country\G
*************************** 1. row ***************************
       Table: country
Create Table: CREATE TABLE `country` (
`country_id` smallint(5) unsigned NOT NULL auto_increment,
`country` varchar(50) NOT NULL,
`last_update` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,
PRIMARY KEY (`country_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.01 sec)

mysql> show create table city\G
*************************** 1. row ***************************
       Table: city
Create Table: CREATE TABLE `city` (
`city_id` smallint(5) unsigned NOT NULL auto_increment,
`city` varchar(50) NOT NULL,
`country_id` smallint(5) unsigned NOT NULL,
`last_update` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,
PRIMARY KEY (`city_id`),
KEY `country_id` (`country_id`),
CONSTRAINT `city_ibfk_1` FOREIGN KEY (`country_id`) REFERENCES `country` (`country_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
mysql> select * from city;
+---------+----------+------------+---------------------+
| city_id | city     | country_id | last_update         |
+---------+----------+------------+---------------------+
|       1 | hancheng |          1 | 2012-01-09 09:18:33 |
+---------+----------+------------+---------------------+
1 row in set (0.01 sec)

mysql> select * from country;
+------------+---------+---------------------+
| country_id | country | last_update         |
+------------+---------+---------------------+
|          1 | chen    | 2012-01-09 09:16:38 |
+------------+---------+---------------------+

mysql> update country set country_id=100 where country_id=1;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`test/city`, CONSTRAINT `city_ibfk_1` FOREIGN KEY (`country_id`) REFERENCES `country` (`country_id`))

上面的问题是说因为有关联的存在，所以无法改变country_id这个字段。
然后自己又重新看了下书本，发现自己的sql语句中没有innodb的外键约束方式（cascade,set null,no action,restrict），感觉这就是自己出问题的地方。
可是怎么加入关联方式呢，上网找了好半天也没有合适的方法。就自己找呗，就通过老师说的方法，？help一点儿一点儿终于找到了怎么改变的方法，文档功能很强大啊

| ADD {INDEX|KEY} [index_name] [index_type] (index_col_name,...)
| ADD [CONSTRAINT [symbol]]
        PRIMARY KEY [index_type] (index_col_name,...)
| ADD [CONSTRAINT [symbol]]
        UNIQUE [INDEX|KEY] [index_name] [index_type] (index_col_name,...)
写了后又是一大堆的错误，无从下手啊

mysql> alter table city add CONSTRAINT `city_ibfk_1` FOREIGN KEY (`country_id`) REFERENCES `country` (`country_id`) ON UPDATE CASCADE;
ERROR 1005 (HY000): Can't create table '.\test\#sql-ed0_37.frm' (errno: 121)
zhouqian@zhou:~$ perror 121
OS error code 121: Remote I/O error
MySQL error code 121: Duplicate key on write or update

Can't create table 'test.icity' (errno: 150)

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mysql中怎么实现外键关联

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