Determine Group By from Parameter - - tsql

Question

How is it that this is valid:
ALTER PROCEDURE [StoredProcedure]
#abcID int = null -- optional param
SELECT columnJ, columnK, Count(eID) AS Num, Sum(OutXYZ) as TotalProdXYZ, Sum(RawXYZ) as TotalRawXYZ
FROM [v_ViewTable]
WHERE (#abcID IS NULL OR (abcID = #abcID))
GROUP BY columnJ, columnK
But then this is throwing a "columnJ is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause."
ALTER PROCEDURE [StoredProcedure]
#abcID int = null -- optional param
-- if 0: Group by columnJ, columnK
-- else: Group by columnK, columnJ
, #Grouping int = null
SELECT columnJ, columnK, Count(eID) AS Num, Sum(OutXYZ) as TotalProdXYZ, Sum(RawXYZ) as TotalRawXYZ
FROM [v_ViewTable]
WHERE (#abcID IS NULL OR (abcID = #abcID))
GROUP BY
CASE WHEN #Grouping = 0 THEN columnJ ELSE columnK END
,CASE WHEN #Grouping = 0 THEN columnK ELSE columnJ END
What's wrong with my CASE statement? Basically, if #Grouping = 0, I want the group by to be J, K if anything else, it should be K, J
Thanks in advance for any help!

Answer 1

As the message says columnJ and columnK are not contained in either an aggregate function or the GROUP BY clause in the second case. You should use field or the exact expression in select list AS IS in the GROUP BY section.
So the following statement will be ok:
SELECT CASE WHEN #Grouping = 0 THEN columnJ ELSE columnK END,
CASE WHEN #Grouping = 0 THEN columnK ELSE columnJ END,
Count(eID) AS Num, Sum(OutXYZ) as TotalProdXYZ, Sum(RawXYZ) as TotalRawXYZ
FROM [v_ViewTable]
WHERE (#abcID IS NULL OR (abcID = #abcID))
GROUP BY
CASE WHEN #Grouping = 0 THEN columnJ ELSE columnK END
,CASE WHEN #Grouping = 0 THEN columnK ELSE columnJ END