How to calculate Running Multiplication

Question

I have two tables

WAC table

ID  wac_inc             item
--  -----------------   ----
1   2.310000000000000   A
2   1.10000000000000         


        

Answer 1

For the sake of completeness here is a full solution for SQL Server 2012 that uses the EXP(SUM(LOG(val))) trick suggested by @usr in another answer.

WITH
CTE
AS
(
    SELECT
        0 AS ID
        ,item
        ,baseline AS wac_inc
        ,baseline AS m
    FROM baseline

    UNION ALL

    SELECT
        ID
        ,item
        ,wac_inc
        ,1 + wac_inc/100 AS m
    FROM wac
)
SELECT
    ID
    ,item
    ,wac_inc
    ,m
    ,EXP(SUM(LOG(m)) OVER (PARTITION BY item ORDER BY ID ROWS UNBOUNDED PRECEDING)) AS MulRows
FROM CTE;

result set

ID  item wac_inc            m                   MulRows
0   A   10.000000000000000  10.000000000000000  10
1   A   2.310000000000000   1.023100000000000   10.231
2   A   1.100000000000000   1.011000000000000   10.343541
3   A   2.130000000000000   1.021300000000000   10.5638584233
4   A   1.340000000000000   1.013400000000000   10.7054141261722

If SQL Server 2012 is available, this window SUM is very efficient. For previous versions any set-based solution would result in O(n*n) complexity, which means that cursor would be a better way. Here is a very good article by Aaron Bertrand comparing different methods of calculating running totals: http://sqlperformance.com/2012/07/t-sql-queries/running-totals Or SO question: Calculate running total / running balance

Of course, if your table is small, then set-based solution with O(n*n) complexity may run faster than O(n) solution with the cursor due to cursor overhead, so you need to check the performance with your real data.