Question

所以我有2张桌子

地方

PlaceID   varchar  PK
PlaceName varchar

行程

TripID      int   PK
Depart      TIME
DepartPlc   varchar
ArrivalPlc  varchar
ArrivalTime TIME

DepartPlc和ArrivalPlc都存储了PlaceID。

我想从旅行中选择数据，但显示与PlaceName和DepartPlc中存储的ID相对应的de ArrivalPlc数据。

任何人都可以帮助我吗？

我目前有这样的陈述：

SELECT TripID, Depart, PlaceName, ArrivalPlc, ArrivalTime 
FROM Trips, Places 
WHERE TripID = 'VALUE' AND PlaceName = DepartPlc;

这就像我想要的那样，但当我为ArrivalPlc添加相同的东西显然不起作用...

Answer 1

使用两个不同的表别名来加入两次

SELECT DPlaces.placeid D_PlaceID, 
       Aplaces.placeid A_PlaceID 
FROM   trips t 
       INNER JOIN places DPlaces 
               ON t.departplc = DPlaces.placename 
       INNER JOIN places APlaces 
               ON t.arrivalplc = aplaces.placename 
WHERE t.TripID = 'VALUE'

Answer 2

您需要在places子句中使用from表两次：

select 
    tripId, depart, 
    departPlc as departPlaceId, dp.placeName as departPlaceName,
    arrivalPlc as arravalPlaceId, ap.placeName as arrivalPlaceName
from
    trips as t
    inner join places as dp on t.departPlc = dp.placeId
    inner join places as ap on t.arrivalPlc = ap.placeId
where
    t.tripId = 'VALUE`

请注意，为了清楚起见，我将关系条件从where子句移动到from子句（ussing内连接）。这样你就可以保持清晰的分离：关系和过滤器。

Answer 3

您需要在Trips上进行第二次加入

SELECT t.TripID, t.Depart, d.PlaceName AS DepartPlaceName, a.PlaceName AS ArrivalPlaceName, t.ArrivalTime
FROM Trips t, Places d, Places a
WHERE t.TripID = 'VALUE' AND t.DepartPlc = d.PlaceID AND t.ArrivalPlc = a.PlaceID

SQL是可能的......它可能是

3 个答案: