如何只获得表连接的第一个结果？

Question

我有一个问题:

SELECT  

    Segment_ID = Segment_ID.Segment_ID,
    Sprav_093.Name as Road_Wear

FROM dbo.Road
LEFT JOIN Segment_ID  ON Road.Road_ID = Segment_ID.Road_ID
LEFT JOIN Driveway_Pavement ON Segment_ID.Segment_ID = Driveway_Pavement.Segment_ID
LEFT JOIN Sprav_093 ON Driveway_Pavement.Kod_Spr093 = Sprav_093.Kod_Spr

工作正常.有几个行与同一Segment_ID表Driveway_Pavement.所以我想要获得这些行中的第一行.我该怎么做？

Answer 1

假设您要为每个段ID返回一行,请执行此操作

;WITh CTE AS (
SELECT  

    Segment_ID = Segment_ID.Segment_ID,
    Sprav_093.Name as Road_Wear,
    CASE WHEN Sprav_093.Name IS NOT NULL THEN 
    ROW_NUMBER() OVER (PARTITION BY Segment_ID.Segment_ID 
                       ORDER BY Sprav_093.Name//or date column) 
    ELSE 1 END rn

FROM dbo.Road
LEFT JOIN Segment_ID  ON Road.Road_ID = Segment_ID.Road_ID
LEFT JOIN Driveway_Pavement ON Segment_ID.Segment_ID = Driveway_Pavement.Segment_ID
LEFT JOIN Sprav_093 ON Driveway_Pavement.Kod_Spr093 = Sprav_093.Kod_Spr
)
SELECT * FROM CTE WHERE RN = 1