SQL capped, continuous sum

Question

Assume a time series where we have one value per point in time. I have to calculate a continues sum by calculating the current value + the sum of the previous value. Tricky part however is, that the sum should be capped and hence not exceed a certain value.

Example: Sum is capped at +2.

+-----+---------+------------+
| row | measure | capped sum |
+-----+---------+------------+
|   1 |       1 |          1 |
|   2 |       3 |          2 |
|   3 |       4 |          2 |
|   4 |      -2 |          0 |
|   5 |       1 |          1 |
+-----+---------+------------+

e.g. the "capped sum" of row 4 is calulated using the "capped sum" from the previous row, "2", and adding the current value. As the result is < 2 we can take the value as it is.

Problem is, with HANA, I cannot use a LAG/Window function on the "capped sum" field. It will give me a "unknown column" error.

Any ideas how to resolve this in SQL/HANA SQL Script without using for loops (which would be very slow)?

Answer 1

This solution uses a recursive-cte which isn't supported in HANA (as per the OP). Posting a solution which works with databases that support it.

WITH ROWNUMS AS
 (SELECT T.*,
         ROW_NUMBER() OVER(ORDER BY ROW) AS RNUM
  FROM T) 
,RCTE AS
 (SELECT ROW,
         RNUM,
         MEASURE,
         MEASURE AS CAPPED_SUM
  FROM ROWNUMS
  WHERE RNUM=1
   UNION ALL
   SELECT RN.ROW,
          RN.RNUM,
          RN.MEASURE,
          CASE
              WHEN R.CAPPED_SUM+RN.MEASURE>=2 THEN 2
              ELSE R.CAPPED_SUM+RN.MEASURE
          END
   FROM ROWNUMS RN
   JOIN RCTE R ON R.RNUM=RN.RNUM-1 )
SELECT ROW,
       MEASURE,
       CAPPED_SUM
FROM RCTE

Sample Demo

Answer 2

This script first creates a column of a running sum. Then it uses that column to create a column of "overage", how much the running sum exceeds the capped value, cumulatively. Then it subtracts the overage in order to give a value less than 2 if appropriate.

DECLARE @capped_value INT = 2
;WITH CTE AS
(SELECT rowID,measure,
    running_total = SUM(measure) OVER 
    (ORDER BY rowID ROWS UNBOUNDED PRECEDING)
FROM dbo.test_capped_sum)
,
CTE2 AS
(SELECT *,
    overage_total = MAX(CTE.running_total) 
    OVER (ORDER BY rowID ROWS UNBOUNDED PRECEDING) - @capped_value
FROM CTE)

SELECT rowid,measure,
    CASE WHEN CTE2.overage_total > 0
    THEN CTE2.running_total- CTE2.overage_total 
    ELSE CTE2.running_total END
    AS capped_sum FROM CTE2