如何執行基於外鍵

Question

這裏一個SQL計數簡化架構：

CREATE TABLE IF NOT EXISTS `person` (
    `Id` int(11) NOT NULL AUTO_INCREMENT, 
    `HousingId` int(11) NOT NULL, 
    PRIMARY KEY (`Id`) 
); 

CREATE TABLE IF NOT EXISTS `housing` (
    `Id` int(11) NOT NULL AUTO_INCREMENT, 
    `Capacity` int(11) NOT NULL, 
    PRIMARY KEY (`Id`) 
); 

我怎麼能簡單地獲得每個房屋的自由空間？

我嘗試以下查詢：

select housing.Id, housing.Capacity - count(person.Id) 
from housing, person 
where person.HousingId = housing.Id 
group by housing.Id 

但它僅適用於具有至少一個人，而不是空的住房。

感謝您的幫助！

Answer 1

使用外部聯接

select housing.Id, housing.Capacity - count(person.Id) 
from housing 
LEFT OUTER JOIN person ON person.HousingId = housing.Id 
group by housing.Id, housing.Capacity