今兒繼續再看老師給推薦的深入淺出mysql數據庫開發這本書，看到innodb數據庫的外鍵關聯問題時，遇到了一個問題，書上寫的是可以對父表進行修改，從而同步到子表的外鍵上去，可是自己的實驗卻是沒有能夠。
復制代碼代碼如下:
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 |
+------------+---------+---------------------+
復制代碼代碼如下:
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`))
上面的問題是說因為有關聯的存在，所以無法改變country_id這個字段。
然后自己又重新看了下書本，發現自己的sql語句中沒有innodb的外鍵約束方式（cascade,set null,no action,restrict），感覺這就是自己出問題的地方。
可是怎么加入關聯方式呢，上網找了好半天也沒有合適的方法。就自己找唄，就通過老師說的方法，？ help一點兒一點兒終于找到了怎么改變的方法，文檔功能很強大啊
復制代碼代碼如下:
| 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,...)
寫了后又是一大堆的錯誤，無從下手啊
復制代碼代碼如下:
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)-----我這里也建立索引了。網上的說法是：字段類型和外鍵的索引
這里是重新建立一張表icity，結果可以了，總結可能是因為字段類型的問題，可是我的alter的問題還是沒有解決呢：
復制代碼代碼如下:
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（老師和網友）的幫助下終于搞定了，做法先drop掉表里的外鍵，然后在add。呵呵……
復制代碼代碼如下:
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)
