Just for fun, using the SQL 2012 LAG function, this query matches ChrisM's first result:
SELECT LinkingID, ID1, ID2
,rn=CASE WHEN ID3=ID2 AND ID4=ID1 THEN 2 ELSE 1 END
FROM
(
SELECT LinkingID, ID1, ID2
,ID3=LAG(ID1) OVER (PARTITION BY LinkingID ORDER BY ID1)
,ID4=LAG(ID2) OVER (PARTITION BY LinkingID ORDER BY ID1)
FROM #Sample
) a;
Using his sample data of course.
My thought question: Have you ever been told that your query runs too fast?
My advice:
INDEXing a poor-performing query is like putting sugar on cat food. Yeah, it probably tastes better but are you sure you want to eat it?
The path of least resistance can be a slippery slope. Take care that fixing your fixes of fixes doesn't snowball and end up costing you more than fixing the root cause would have in the first place.
Need to UNPIVOT? Why not CROSS APPLY VALUES instead?[/url]
Since random numbers are too important to be left to chance, let's generate some![/url]
Learn to understand recursive CTEs by example.[/url]
[url url=http://www.sqlservercentral.com/articles/St