SQL 无法获取准确数据

Question

我在 SQL 中获取数据时遇到问题。如何拥有一个 table 源或主键但有两个不同的外键。

This is the ERD This is the result but when i put the Departure it will be vice versa

Answer 1

如果我没有正确理解您的问题，那么您似乎是在尝试从同一个连接中获取 "Departure" 和 "Arrival" 值。为了使其正常工作，您需要为 "Departure" 和 "Arrival" 中的每一个分别创建两个从 Routes 到 Airports 的单独连接（并相应地使用别名）。

select date, 
    time, 
    ArrivalAirport.IATACode as Departure,
    DepartureAirport.IATACode as Arrival,
    FlightNumber,
    Aircrafts.Name,
    EconomyPrice,
    Schedules.ID
from Schedules
inner join [Routes] on Schedules.RouteID = [Routes].ID
inner join Aircrafts on Schedules.AircraftID = Aircrafts.ID
inner join Airports as ArrivalAirport on ArrivalAirport.ID = [Routes].ArrivalAirportID
inner join Airports as DepartureAirport on DepartureAirport.ID = [Routes].DepartureAirportID

请注意，"Airports" 连接的两个实例需要相应地为它们设置别名，以便您可以在 select 投影中执行所需的操作。

Answer 2

您需要为使用两次的 table 使用 table 别名，然后加入它两次。

SELECT Routes.ID, DEP.IATACode, ARR.IATACode
FROM dbo.Routes
INNER JOIN dbo.Airports AS DEP ON DEP.ID = Routes.DepartureAirportID
INNER JOIN dbo.Airports AS ARR ON ARR.ID = Routes.ArrivalAirportID

一旦定义，您就可以在查询中的任何地方使用别名。参见 here for more on using an alias

Answer 3

以下是我认为您正在寻找的内容：

SELECT
    Schedules.Date,
    Schedules.Time,
    DepartureAirport.IATACode AS Departure,
    ArrivalAirport.IATACode AS Arrival,
    Schedules.FlightNumber,
    Aircrafts.Name,
    Schedules.EconomyPrice,
    Schedules.ID
FROM
    Schedules
    INNER JOIN Aircrafts ON Schedules.AircraftID = Aircrafts.ID
    INNER JOIN Routes ON Schedules.RouteID = Routes.ID
    INNER JOIN Airports AS DepartureAirport 
            ON Routes.DepartureAirportID = DepartureAirport.ID
    INNER JOIN Airports AS ArrivalAirport
            ON Routes.ArrivalAirportID = DepartureAirport.ID

问题是有两个单独的连接到机场 table 这样你就可以有两个不同的机场。否则你的航班会很短，乘客会很不满意。

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