MYSQL：由兩個外鍵從同一個表中選擇

Question

我有以下表格：

航班： ID，idcompany，idplane，fromCity，toCity，起飛......

公司： ID，名稱

面： ID，名稱

城市： ID，名稱

我想在同一個查詢中聲明兩個城市的名稱「fromCity，toCity」。

我的這個行動的目的是一次獲得城市名稱，所以我可以將它們展示給客戶端，而不需要做另一個查詢來獲取城市名稱。

這裏是我的嘗試：

SELECT f.id, f.takeoff, f.arrival, ct.name as fromCity, f.toCity, c.name as company, p.name as plane 
FROM flights f, companies c, planes p, cities ct 
WHERE f.idCompany = c.id AND f.idPlane = p.id AND f.fromCity = ct.id 
ORDER BY f.takeoff ASC 

此查詢將返回「fromCity」，但「toCity」的id的名字，所以我能做些什麼用相同的查詢，以獲得兩個名字？

Answer 1

您可以使用此：

SELECT 
    f.id, f.takeoff, f.arrival, ct1.name as fromCity 
    ,ct2.name as toCity, c.name as company, p.name as plane 
FROM 
    flights f 
inner join 
    companies c 
on 
    f.idCompany = c.id 
inner join 
    planes p 
on 
    f.idPlane = p.id 
inner join 
    cities ct1 
on 
    f.fromCity = ct1.id 
inner join 
    cities ct2 
on 
    f.toCity = ct2.id 
ORDER BY 
    f.takeoff ASC; 

並嘗試不使用old-style-join從現在開始。

Answer 2

你必須加入表格兩次。

SELECT f.id, f.takeoff, f.arrival, ct.name as fromCity, f.toCity, c.name as company, p.name as plane , cf.name 
FROM flights f, companies c, planes p, cities ct,cities cf 
WHERE f.idCompany = c.id AND f.idPlane = p.id AND f.fromCity = ct.id AND f.toCity = cf.id 
ORDER BY f.takeoff ASC 

Answer 3

我的解決方案是使表城市的兩個副本：

SELECT f.id, f.takeoff, f.arrival, ct1.name as fromCity, ct2.name as toCity, c.name as company, p.name as plane 

FROM flights f, companies c, planes p, cities ct1, cities ct2 

WHERE f.idCompany = c.id AND f.idPlane = p.id AND f.fromCity = ct1.id AND ct2.id = f.toCity 

ORDER BY f.takeoff ASC