sql - 如何在表连接中仅获得第一个结果？

Question

我有一个查询：

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。Segment_ID表中有几行相同Driveway_Pavement。所以我想得到这行的第一行。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

Answer 2

尝试将 MAX 函数应用于两个选定的字段

Answer 3

这应该有效：

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

如果你想要一个特定的“第一”行，你可以ORDER BY在SELECT里面添加一个。OUTER APPLY