Question

I have the following query which as you can see does multiple Count(CompetitorID) calls. Is this a performance issue, or does SQL Server 2008 ‘cache’ the Count? If this is a performance issue, is it possible to store the Count to prevent the multiple lookups?

SELECT EventID,Count(CompetitorID) AS NumberRunners,
    CASE WHEN Count(CompetitorID)<5 THEN 1
         WHEN Count(CompetitorID)>=5 AND Count(CompetitorID)<=7 THEN 2
         ELSE 3 END AS NumberPlacings
FROM Comps
GROUP BY EventID Order By EventID;

Answer 1

Its always a better practice to get the value once and use it subsequently whenever possible. In your case, you can always use Inner query to get the count only once and compute other (derived) columns off its value as shown below:

SELECT  EventID, NumberRunners,
        CASE WHEN NumberRunners <5 THEN 1
             WHEN NumberRunners >=5 AND NumberRunners <=7 THEN 2
             ELSE 3 
        END AS NumberPlacings
  FROM  (
            SELECT  EventID,
                    NumberRunners = Count(CompetitorID)
              FROM  Comps
            GROUP BY EventID
        ) t
Order By EventID;