Question

我的数据库中有两张表：

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

如果我在各个国家/地区获得了美国，加拿大，德国和荷兰，并希望将其分组：

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

如何在我的数据库中保存？如何将同一个国家/地区保存在一个以上的组中？如果每个国家只有一个组，只需在国家/地区添加group_id就可以轻松实现。

提前致谢，的Mikael

Answer 1

这是一对多关系。在关系数据库中，这由关联表表示，该表具有（至少）两列 -

country_groups ( country_id, group_id )

然后，当涉及到查询时，您将包含联接以引入此表，例如，对于北美的所有国家/地区：

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'

Answer 2

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

如何保存国家/地区分配到哪个组？

2 个答案: