Question

我在Mysql数据库中有三个表 - 国家，城市和酒店。他们的关系是：国家有很多城市和城市都有很多酒店。

我希望能够选择给定某些国家/地区的所有酒店。我在SQL方面很弱。我想出的连接语句如下，总是返回0行。

SELECT 
    countries.name as country,
    cities.name as city,
    hotels.name as hotel
FROM 
    countries
    left join cities
        on countries.id = cities.country_id
    left join hotels
        on cities.id = hotels.city_id

WHERE
    countries.id = @id

帮助？

Answer 1

尝试离开 off where子句并查看实际所拥有的记录。也许您使用的具体ID不是已知的国家/地区ID。

另外，仅仅是严格的格式/样式问题，恕我直言，这是更好的：

SELECT 
    countries.name as country,
    cities.name as city,
    hotels.name as hotel
FROM countries
left join cities on cities.country_id = countries.id
left join hotels on hotels.city_id = cities.id
WHERE countries.id = @id

Answer 2

我的经验法则是始终使用我想要的实际表格来启动我的FROM子句。

SELECT 
  countries.name as country,
  cities.name as city,
  hotels.name as hotel
FROM  
  hotels JOIN cities ON cities.id = hotels.city_id
  join countries on countries.id = cities.country_id
WHERE
  countries.id = @id

额外的好处是你不需要外连接： - ）

如何加入三个半相关表

2 个答案: