Unexpected Results from PROC SQL Left Join

[Ksharp]

That would be extra helpful if you could post some real data to replicate this problem.

And Kurt gave you the point that missing value is still a key value in SAS.
And if it was 1:1 or 1:n matched ,that would be the same result for SQL and MERGE. But if it was n:n matched, that woud get different result.