Obtaining only first result from a LEFT JOIN - sql

Question

I'm trying to get the first result of a LEFT JOIN for each row of a SELECT statement.
Because now right now, if I have 100 rows in the joined table, I'll get 100 times the same row from the SELECT. I'd just need the first joined row so that way I wouldn't get any duplicates.
I can't use GROUP BY because I have to get more than only one row from the table.
Here's a basic version of my query:
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
I tried using DISTINCT but since the data from bg.CreateDate isn't always the same it returns duplicates.
I just need the FIRST row of that left joined table.
Any ideas/suggestions?
Thanks!

Answer 1

;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;
To join to other results:
;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;

Answer 2

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
Cross apply is a handy tool for such situations. It works like a join but you can use variables inside the subquery.