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

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.