如何保存一个国家被分配到哪个组？

Question

我在我的DB两个表：

countries (id, name) 
groups (group_id, name, countries) 

如果我得到了美国，加拿大，德国和荷兰的国家，并希望他们这样的组群：

USA, Canada = North America 
Germany, Holland = Europe 
USA, Canada, Germnay, Holland = The World 

如何能够而且将会我保存在我的数据库？ 如何将同一个国家保存在多个组中？ 如果每个国家只有一个组，那么只需在国家中添加group_id即可。

由于提前， 的Mikael

Answer 1

示例数据库设计：

CREATE TABLE Countries 
    (`id` INT auto_increment primary key , `name` varchar(30)) 
; 

INSERT INTO Countries 
    (`name`) 
VALUES 
('Germany'),('USA'),('Holland'),('Canada') 

; 

CREATE TABLE Groups_Countries 
(`id` INT auto_increment primary key , Countries_ID int, Groups_id int) 
; 

INSERT INTO Groups_Countries 
    (Countries_id,Groups_id) 
VALUES 
(2,1),(4,1),(1,2),(3,2),(1,3),(2,3),(3,3),(4,3)  
; 



CREATE TABLE Groups 
(`id` INT auto_increment primary key , `name` varchar(30)) 
; 

INSERT INTO Groups 
    (`name`) 
VALUES 
('North America'),('Europe'),('The World') 
; 

SQL查询

SELECT c.name FROM Countries c 
JOIN Groups_Countries gc 
ON c.ID = gc.countries_ID 
JOIN Groups g 
ON g.id = gc.Groups_ID 
WHERE g.name = 'North America'; 

SELECT c.name FROM Countries c 
JOIN Groups_Countries gc 
ON c.ID = gc.countries_ID 
JOIN Groups g 
ON g.id = gc.Groups_ID 
WHERE g.name = 'Europe'; 

SELECT c.name FROM Countries c 
JOIN Groups_Countries gc 
ON c.ID = gc.countries_ID 
JOIN Groups g 
ON g.id = gc.Groups_ID 
WHERE g.name = 'The World'; 

SQLFiddle demo

Answer 2

这是一个多对多的关系。在关系数据库中，这是通过关联表，其中有（至少）表示两列 -

country_groups (country_id, group_id) 

然后，当它涉及到进行查询，你将包括联接在这个表带上，例如对所有国家北美：

SELECT c.name FROM countries c 
    INNER JOIN country_groups cg ON cg.country_id = c.id 
    INNER JOIN groups g ON cg.group_id = g.group_id 
    WHERE g.name = 'North America'