深入mysql外键关联问题的详解--Java学习网

今儿继续再看老师给推荐的深入浅出mysql数据库开发这本书，看到innodb数据库的外键关联问题时，遇到了一个问题，书上写的是可以对父表进行修改，从而同步到子表的外键上去，可是自己的实验却是没有能够。

代码如下:

mysql> 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 | +------------+---------+---------------------+

代码如下:

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

代码如下:

| 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

代码如下:

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)-----我这里也建立索引了。网上的说法是：字段类型和外键的索引

这里是重新建立一张表icity，结果可以了，总结可能是因为字段类型的问题，可是我的alter的问题还是没有解决呢：

代码如下:

mysql> create table icity(id int not null, city varchar(20), country_id smallint unsigned not null , primary key(id), foreign key(country_id) references country(country_id) on update cascade )engine=innodb; Query OK, 0 rows affected (0.11 sec) mysql> show create table icity\G *************************** 1. row *************************** Table: icity Create Table: CREATE TABLE `icity` ( `id` int(11) NOT NULL, `city` varchar(20) DEFAULT NULL, `country_id` smallint(5) unsigned NOT NULL, PRIMARY KEY (`id`), KEY `country_id` (`country_id`), CONSTRAINT `icity_ibfk_1` FOREIGN KEY (`country_id`) REFERENCES `country` (`country_id`) ON UPDATE CASCADE ) ENGINE=InnoDB DEFAULT CHARSET=latin1 1 row in set (0.02 sec)

在大家（老师和网友）的帮助下终于搞定了，做法先drop掉表里的外键，然后在add。呵呵……

代码如下:

mysql> alter table city drop FOREIGN KEY `city_ibfk_1`; Query OK, 0 rows affected (0.24 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> alter table city add FOREIGN KEY (`country_id`) REFERENCES `country` (`country_id`) ON UPDATE CASCADE;Query OK, 0 rows affected (0.16 sec) Records: 0 Duplicates: 0 Warnings: 0 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`), KEY `idx_fk_country_id` (`country_id`), CONSTRAINT `city_ibfk_1` FOREIGN KEY (`country_id`) REFERENCES `country` (`country_id`) ON UPDATE CASCADE ) ENGINE=InnoDB DEFAULT CHARSET=utf8 1 row in set (0.00 sec)