Using a custom aggregate function in a GROUP BY?

Question

I have a simple MEDIAN calculation function:

IF OBJECT_ID(N'COMPUTEMEDIAN', N'FN') IS NOT NULL
    DROP FUNCTION dbo.COMPUTEMEDIAN;
GO
CREATE FUNCTION dbo.COMPUTEMEDIAN(@VALUES NVARCHAR(MAX))
RETURNS DECIMAL
WITH EXECUTE AS CALLER
AS
BEGIN
    DECLARE @SQL NVARCHAR(MAX)
    DECLARE @MEDIAN DECIMAL
    SET @MEDIAN = 0.0;

    DECLARE @MEDIAN_TEMP TABLE (RawValue DECIMAL);

    -- This is the Killer!
    INSERT INTO @MEDIAN_TEMP
    SELECT s FROM master.dbo.Split(',', @VALUES) OPTION(MAXRECURSION 0)  

    SELECT @MEDIAN =
    (
     (SELECT MAX(RawValue) FROM
       (SELECT TOP 50 PERCENT RawValue FROM @MEDIAN_TEMP ORDER BY RawValue) AS BottomHalf)
     +
     (SELECT MIN(RawValue) FROM
       (SELECT TOP 50 PERCENT RawValue FROM @MEDIAN_TEMP ORDER BY RawValue DESC) AS TopHalf)
    ) / 2

    --PRINT @SQL
    RETURN @MEDIAN;
END;
GO

However, my table is of the following form:

CREATE TABLE #TEMP (GroupName VARCHAR(MAX), Value DECIMAL)
INSERT INTO #TEMP VALUES ('A', 1.0)
INSERT INTO #TEMP VALUES ('A', 2.0)
INSERT INTO #TEMP VALUES ('A', 3.0)
INSERT INTO #TEMP VALUES ('A', 4.0)
INSERT INTO #TEMP VALUES ('B', 10.0)
INSERT INTO #TEMP VALUES ('B', 11.0)
INSERT INTO #TEMP VALUES ('B', 12.0)

SELECT * FROM #TEMP

DROP TABLE #TEMP

What is the best way to invoke the MEDIAN function on this table using a GROUP BY on the id column? So, I am looking for something like this:

SELECT id, COMPUTEMEDIAN(Values)
FROM #TEMP
GROUP BY id

My current approach involves using XMLPATH to combine all values resulting from a GROUP BY operation into a large string and then passing it to the function but this involves the String splitting operation and for large strings this just slows down everything. Any suggestions?

Answer 1

Since you're using SQL Server 2008, I would suggest writing the aggregate function as a CLR function.

http://msdn.microsoft.com/en-us/library/91e6taax(v=vs.80).aspx

Also, people have asked this question before. Perhaps their answers would be helpful

Function to Calculate Median in Sql Server