如何只获得表联接中的第一个结果？

Question

I have a query:

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

Is works fine. There are several row with same Segment_ID in table Driveway_Pavement . So i want get just first of this rows. How can i do it?

Answer 1

Assuming that you want to return one row for each segment id, do this

;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

Answer 2

尝试将MAX函数应用于两个选定字段

Answer 3

This should work:

SELECT  

    Segment_ID = Segment_ID.Segment_ID,
    Sprav_093.Name as Road_Wear

FROM dbo.Road
OUTER APPLY (
    SELECT TOP 1 Segment_ID.Segment_ID
    FROM Segment_ID
    WHERE Segment_ID.Road_ID = Road.Road_ID
) Segment_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

You can add an ORDER BY to the SELECT inside of the OUTER APPLY if you want a specific "first" row.