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Question Abdul-Rashid Yakubu · Mar 22, 2022

Finding the median in intersystems cache SQL

#Caché #SQL

Hi,

Is there a way to find the median in Intersystems Cache SQL? I know it is not available as an aggregate function. Also in SQL Server I could try something like: 

SELECT
(
 (SELECT MAX(Score) FROM
   (SELECT TOP 50 PERCENT Score FROM Posts ORDER BY Score) AS BottomHalf)
 +
 (SELECT MIN(Score) FROM
   (SELECT TOP 50 PERCENT Score FROM Posts ORDER BY Score DESC) AS TopHalf)
) / 2 AS Median

However, there is no PERCENT Keyword in Cache as well. Any suggestions?

Thanks

Product version: Caché 2012.1
Discussion (4)1
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Comments

Benjamin De Boe · Apr 8, 2022

As of IRIS 2021.1, we allow users to create their own aggregate functions. Perhaps there's a beautiful community contribution in there? :-)

You could build something simple where you just stuff all values in a temporary global (the name of which you pass as a state) and sort them (using $sortbegin/$sortend), maintaining a count and then in the FINALIZE method gather the actual median value.

Two caveats:

  • don't bother implementing a MERGE function. We don't support parallel execution just yet.
  • in some query execution plans, the FINALIZE method may be called more than once (e.g. if the aggregate is used in the SELECT list and say a HAVING clause). So you may want to cache your result somewhere (a PPG will do as this is in the last single-process mile of query processing, typically mere milliseconds apart)

We'll be removing these annoyances in a future version

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Randy Shaw · Apr 29, 2022

SELECT TOP 1

AVG(main.age) AS _Average,min(main.age) AS _Min,

CASE WHEN %vid = count(main.age)/2 THEN main.age else 0 END+MAX(CASE WHEN %vid = count(main.age)/2 THEN main.age else 0 END) AS _Median,

max(main.age) AS _Max

FROM ( SELECT TOP all a.Age FROM Sample.Person a ORDER BY a.Age ) main

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