两个表之间的两个连接

Question

我有两个表来存储地址，如下所示：

我想获取每个地址，城市，名称和地区名称（cityId，Areaid可能为null）

我试过了：

SELECT [Details]
      ,[AddressId]
      ,[CityId]
      ,[PlaceName] as CityName
      ,[AreaId]
      ,[PlaceName] as AreaName
  FROM [MyDB].[dbo].[Address] LEFT OUTER JOIN [MyDB].[dbo].[Places]
  ON [CityId] = [PlaceI]  

但这只会给我这个城市的名字。如何获得区域名称？

Answer 1

您需要使用Places表格加入两次。一次是城市，一次是区域。

SELECT [Details]
      ,[AddressId]
      ,[CityId]
      ,C.[PlaceName] as CityName
      ,[AreaId]
      ,A.[PlaceName] as AreaName
FROM [MyDB].[dbo].[Address] 
     LEFT OUTER JOIN [MyDB].[dbo].[Places] C
         ON [CityId] = C.[PlaceI] 
     LEFT OUTER JOIN [MyDB].[dbo].[Places] A
         ON [AreaId] = A.[PlaceI] 

Answer 2

添加其他联接

SELECT [Details]
      ,[AddressId]
      ,[CityId]
      ,p1.[PlaceName] as CityName
      ,[AreaId]
      ,p2.[PlaceName] as AreaName
  FROM [MyDB].[dbo].[Address] 
  LEFT OUTER JOIN [MyDB].[dbo].[Places] p1 ON [CityId] = p1.[PlaceId]  
  LEFT OUTER JOIN [MyDB].[dbo].[Places] p2 ON [AreaId] = p2.[PlaceId] 

Answer 3

试试这个

SELECT [Details]
      ,[AddressId]
      ,[CityId]
      ,C.[PlaceName] as CityName
      ,[AreaId]
      ,A.[PlaceName] as AreaName
FROM [MyDB].[dbo].[Address] 
     INNER JOIN [MyDB].[dbo].[Places] C
     ON ([CityId] = [PlaceID] AND IsCity=1)
     OR  ([AreaId] = [PlaceID] AND IsCity=0)

Answer 4

会在同一个表中添加另一个连接...这次加入区域ID工作？  （其他答案因此而没有添加查询）

您可能希望将另一列添加到要为区域ID加入的位置，或者放入第三个表中，以便在可能的情况下删除多对多关系