仅从左加入获取第一个结果

Question

我试图获得SELECT语句的每一行的第一个结果LEFT JOIN。

因为现在，如果我在连接表中有100行，我会从SELECT获得100倍的同一行。我只需要第一个连接的行，这样我就不会有任何重复。

我不能使用GROUP BY，因为我必须从表格中获取不止一行。

这里是我的查询的基本版本：

SELECT bg.PatientID, DATEDIFF(hour, bg.CreateDate, GETDATE()) TimeToTarget 
FROM BloodGlucose bg 
    LEFT JOIN IVProtocol i ON i.PatientID = bg.PatientID 
WHERE bg.BGValue >= i.TargetLow AND bg.BGValue <= i.TargetHigh 
ORDER BY bg.PatientID ASC 

我使用DISTINCT尝试，但由于从bg.CreateDate数据并不总是相同的它返回重复。

我只需要左连接表的第一行。

任何意见/建议？

谢谢！

Answer 1

SELECT bg.PatientID, DATEDIFF(hour, bg.CreateDate, GETDATE()) TimeToTarget 
FROM BloodGlucose bg 
cross apply (
select top 1 * 
from IVProtocol i 
where i.PatientID = bg.PatientID 
order by SOME_CRITERA 
) i 
WHERE bg.BGValue >= i.TargetLow AND bg.BGValue <= i.TargetHigh 
ORDER BY bg.PatientID ASC 

交叉应用是这种情况的一个方便的工具。它像联接一样工作，但可以在子查询中使用变量。

Answer 2

;WITH x AS 
(
    SELECT 
    bg.PatientID, 
    TimeToTarget = DATEDIFF(hour, bg.CreateDate, GETDATE()), 
    rn = ROW_NUMBER() OVER (PARTITION BY bg.PatientID ORDER BY bg.CreatedDate DESC) 
    FROM dbo.BloodGlucose AS bg 
    LEFT JOIN dbo.IVProtocol AS i 
    ON i.PatientID = bg.PatientID 
    WHERE bg.BGValue >= i.TargetLow 
    AND bg.BGValue <= i.TargetHigh 
) 
SELECT PatientID, TimeToTarget 
FROM x 
WHERE rn = 1 
ORDER BY PatientID; 

要加入到其他结果：

;WITH x AS 
(
    ... same as above ... 
) 
SELECT x.PatientID, x.TimeToTarget, y.Something 
FROM x INNER JOIN dbo.SomethingElse AS y 
ON x.PatientID = y.PatientID 
WHERE x.rn = 1 
ORDER BY x.PatientID;