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Asked: 2026-05-27T21:32:06+00:00

Given a table of user points earned, I need to calculate a six month

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Given a table of user points earned, I need to calculate a six month half-life of the cumulative points earned. That is, all points earned in the past 6 months are valued at 100%, 6-12 months at 50%, 12-18 months %25, etc.

Here is my best attempt using a CTE:

DECLARE @t AS TABLE (
    ID int NOT NULL, 
    DT SMALLDATETIME NOT NULL, 
    POINTS int NOT NULL, 
    UserId INT NOT null, 
    POINTS_TOTAL INT NOT NULL)
INSERT INTO @t 
VALUES (1, '1/1/2010', 20, 1, 20)
    ,(5, '7/1/2010', 30, 1, 50)
    ,(7, '1/1/2011', 10, 1, 60)
    ,(8, '7/1/2011', 15, 1, 75)
    ,(9, '12/25/2011', 15, 1, 90)

;WITH ctePts AS 
    (SELECT t.*, POINTS AS Decay
    FROM @t AS t
    WHERE t.DT > DATEADD(mm, 6, GETUTCDATE())

    UNION ALL

    SELECT t.ID, t.DT, t.POINTS, t.UserId, t.POINTS_TOTAL, CAST(ctePts.Decay * 0.5 + t.POINTS AS INT) AS Decay
    FROM ctePts
    INNER JOIN @t AS t
        ON t.UserId = ctePts.UserId 
            AND t.DT <= ctePts.DT AND t.DT > DATEADD(mm, 6, ctePts.DT)
)
SELECT *
FROM ctePts

The final table would have a HALF_LIFE column that contains the adjusted sum of all points earned for that user. I should be able to run the following query to get the user’s current point status (total accumulated points those adjusted for half-life decay) for any given point in time:

SELECT UserIdInt, POINTS, POINTS_HALFLIFE FROM 
    (SELECT UserIdInt, 
                ISNULL(p.POINTS_TOTAL,0) AS POINTS, ISNULL(p.POINTS_HALFLIFE,0) POINTS_HALFLIFE,
                ROW_NUMBER() OVER (Partition BY UserIdInt ORDER BY p.ID DESC) AS rownum

    FROM  dbo.USER_POINTS p ) a
WHERE rownum = 1
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1 Answer

  1. Editorial Team

    Is this what you need?

    SELECT *,
       POINTS / POWER(2E0, ( ( DATEDIFF(MONTH, DT, GETUTCDATE()) - 1 + 
                           CASE
                             WHEN DAY(DT) <= DAY(GETUTCDATE()) THEN 1
                             ELSE 0
                           END ) / 6 ))
           AS POINTS_HALFLIFE
FROM   @t

    Returns

    ID          DT                      POINTS      UserId      POINTS_TOTAL POINTS_HALFLIFE
----------- ----------------------- ----------- ----------- ------------ ----------------------
1           2010-01-01 00:00:00     20          1           20           2.5
5           2010-07-01 00:00:00     30          1           50           7.5
7           2011-01-01 00:00:00     10          1           60           5
8           2011-07-01 00:00:00     15          1           75           15
9           2011-12-25 00:00:00     15          1           90           15
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