mysql外键关联问题

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。
 
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 | 
+------------+---------+---------------------+ 
 
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。