SQL: Select maximum value for each unique key?

Question

Sorry, I\'m not sure how to phrase that and I\'m really not very good with SQL. The db engine i SQL Server Compact. I currently have this query:

SELECT *
FRO         


        

Answer 1

Here's how I would do it:

SELECT s1.*
FROM Samples s1
LEFT JOIN Samples s2 
  ON (s1.Thread = s2.Thread and s1.HitCount < s2.HitCount)
WHERE s1.FunctionId NOT IN (SELECT CalleeId FROM Callers) 
  AND s2.Thread IS NULL
ORDER BY s1.ThreadId, s1.HitCount DESC

In other words, the row s1 for which there is no other row s2 matching the same Thread and having a greater HitCount.

Answer 2

Will work with SQL Server 2005+:

WITH maxHits AS(
  SELECT s.threadid,
         MAX(s.hitcount) 'maxhits'
    FROM SAMPLES s
    JOIN CALLERS c ON c.threadid = s.threadid AND c.calleeid != s.functionid
GROUP BY s.threadid
)
SELECT t.*
  FROM SAMPLES t
  JOIN CALLERS c ON c.threadid = t.threadid AND c.calleeid != t.functionid
  JOIN maxHits mh ON mh.threadid = t.threadid AND mh.maxhits = t.hitcount

Work on any database:

SELECT t.*
  FROM SAMPLES t
  JOIN CALLERS c ON c.threadid = t.threadid AND c.calleeid != t.functionid
  JOIN (SELECT s.threadid,
               MAX(s.hitcount) 'maxhits'
          FROM SAMPLES s
          JOIN CALLERS c ON c.threadid = s.threadid AND c.calleeid != s.functionid
      GROUP BY s.threadid) mh ON mh.threadid = t.threadid AND mh.maxhits = t.hitcount

Answer 3

Does SQL Server compact support windowed functions?

Alternative 1--Will include all rows that tie. Will not include a row, if the only rows for a given Thread all have null for HitCount:

SELECT Thread, Function, HitCount
FROM (SELECT Thread, Function, HitCount,
        MAX(HitCount) over (PARTITION BY Thread) as MaxHitCount
    FROM Samples
    WHERE FunctionId NOT IN
        (SELECT CalleeId FROM Callers)) t 
WHERE HitCount = MaxHitCount 
ORDER BY ThreadId, HitCount DESC

Alternative 2--Will include all rows that tie. If there is no row for a given thread with non-null HitCount, will return all rows for that thread:

SELECT Thread, Function, HitCount
FROM (SELECT Thread, Function, HitCount,
        RANK() over (PARTITION BY Thread ORDER BY HitCount DESC) as R
    FROM Samples
    WHERE FunctionId NOT IN
        (SELECT CalleeId FROM Callers)) t
WHERE R = 1
ORDER BY ThreadId, HitCount DESC

Alternative 3--Will non-determistically pick one row in case of ties and discard others. Will include a row if all rows for a given thread have null HitCount

SELECT Thread, Function, HitCount
FROM (SELECT Thread, Function, HitCount,
        ROW_NUMBER() over (PARTITION BY Thread ORDER BY HitCount DESC) as R
    FROM Samples
    WHERE FunctionId NOT IN
        (SELECT CalleeId FROM Callers)) t
WHERE R = 1
ORDER BY ThreadId, HitCount DESC

Alternative 4 & 5--Uses older constructs, if the windowed functions aren't available, and says what is meant a little cleaner than using joins. Benchmark if spead is a priority. Both return all rows that participate in a tie. Alternative 4 will HitCount is null when non-null values are not available for HitCount. Alternative 5 will not return rows with HitCount is null.

SELECT *
FROM Samples s1
WHERE FunctionId NOT IN
    (SELECT CalleeId FROM Callers)
AND NOT EXISTS
    (SELECT *
    FROM Samples s2
    WHERE s1.FunctionId = s2.FunctionId
    AND s1.HitCount < s2.HitCount)
ORDER BY ThreadId, HitCount DESC

SELECT *
FROM Samples s1
WHERE FunctionId NOT IN
    (SELECT CalleeId FROM Callers)
AND HitCount = 
    (SELECT MAX(HitCount)
    FROM Samples s2
    WHERE s1.FunctionId = s2.FunctionId)
ORDER BY ThreadId, HitCount DESC