MySQL外键关联（创世纪新篇）

数据库外键

• 01.mysql> show create table country\G 
  02.*************************** 1. row *************************** 
  03.       Table: country 
  04.Create Table: CREATE TABLE `country` ( 
  05.  `country_id` smallint(5) unsigned NOT NULL auto_increment, 
  06.  `country` varchar(50) NOT NULL, 
  07.  `last_update` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP, 
  08.  PRIMARY KEY  (`country_id`) 
  09.) ENGINE=InnoDB DEFAULT CHARSET=utf8 
  10.1 row in set (0.01 sec) 
  11. 
  12.mysql> show create table city\G 
  13.*************************** 1. row *************************** 
  14.       Table: city 
  15.Create Table: CREATE TABLE `city` ( 
  16.  `city_id` smallint(5) unsigned NOT NULL auto_increment, 
  17.  `city` varchar(50) NOT NULL, 
  18.  `country_id` smallint(5) unsigned NOT NULL, 
  19.  `last_update` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP, 
  20.  PRIMARY KEY  (`city_id`), 
  21.  KEY `country_id` (`country_id`), 
  22.  CONSTRAINT `city_ibfk_1` FOREIGN KEY (`country_id`) REFERENCES `country` (`country_id`) 
  23.) ENGINE=InnoDB DEFAULT CHARSET=utf8 
  24.1 row in set (0.00 sec) 
  25.mysql> select * from city; 
  26.+---------+----------+------------+---------------------+ 
  27.| city_id | city     | country_id | last_update         | 
  28.+---------+----------+------------+---------------------+ 
  29.|       1 | hancheng |          1 | 2012-01-09 09:18:33 | 
  30.+---------+----------+------------+---------------------+ 
  31.1 row in set (0.01 sec) 
  32. 
  33.mysql> select * from country; 
  34.+------------+---------+---------------------+ 
  35.| country_id | country | last_update         | 
  36.+------------+---------+---------------------+ 
  37.|          1 | chen    | 2012-01-09 09:16:38 | 
  38.+------------+---------+---------------------+ 
  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 |
  +------------+---------+---------------------+

  
  01.mysql> update country set country_id=100 where country_id=1; 
  02.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`)) 
  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

   

  01.| ADD {INDEX|KEY} [index_name] [index_type] (index_col_name,...) 
  02.  | ADD [CONSTRAINT [symbol]] 
  03.        PRIMARY KEY [index_type] (index_col_name,...) 
  04.  | ADD [CONSTRAINT [symbol]] 
  05.        UNIQUE [INDEX|KEY] [index_name] [index_type] (index_col_name,...) 
  | 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

   

  01.mysql> alter table city add CONSTRAINT `city_ibfk_1` FOREIGN KEY (`country_id`) REFERENCES `country` (`country_id`) ON UPDATE CASCADE; 
  02.ERROR 1005 (HY000): Can‘t create table ‘.\test\#sql-ed0_37.frm‘ (errno: 121) 
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