SQL查询 - IS NULL

Question

表：

Country
-------
PK CountryID

Name

City
-------
PK CityID

FK CountryID

Name

Airport
--------
PK AirportID

FK CityID

Name

我的任务是选择没有机场的国家/地区的名称。

我可以想象只有一个解决方案有EXCEPT（或MINUS）

SELECT Country.Name 
FROM Country EXCEPT (SELECT DISTINCT Country.Name FROM Country, City, Airport 
WHERE City.CountryID = Country.CountryID AND Airport.CityID = City.CityID); 

但是有可能不使用EXCEPT而是像IS NULL那样吗？

Answer 1

SELECT cn.CountryID 
  FROM Country cn
  LEFT JOIN City ct ON cn.CountryID = ct.CountryID
  LEFT JOIN Airport ar on ar.CityID=ct.CityID
 WHERE ar.AirportID is null

Answer 2

如果您需要使用IS NULL进行此查询，请尝试以下查询：

SQLFiddle demo

select ct.CountryId,max(ct.Name) from Country ct
left join City c on ct.CountryId=c.CountryId
left join Airport a on a.CityId=c.CityID
group by ct.CountryId
HAVING max(a.AirportID) IS NULL

Answer 3

你可以使用它，但它与你的语法基本相同。

SELECT Country.Name 
FROM Country 
Where Country.Name Not IN 
    (
        SELECT DISTINCT Country.Name FROM Country, City, Airport 
        WHERE City.CountryID = Country.CountryID AND Airport.CityID = City.CityID
    );