《MYSQL教程深入mysql外键关联问题的详解》要点：
本文介绍了MYSQL教程深入mysql外键关联问题的详解，希望对您有用。如果有疑问，可以联系我们。
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)
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