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

You can transform a series of multiplications into a series of additions with the following math trick:

exp(log(a) + log(b)) = a * b

So MUL(a) is EXP(SUM(LOG(a))).

SELECT SUM(val) AS [Sum], EXP(SUM(LOG(val))) AS Product
FROM (VALUES 
    (1), (2), (3), (4)
) x(val)

This emits sum = 10, product = 24.

Potential problems are rounding errors and zero factors.

You can now use one of the usual ways to achieve a running aggregate such as windowing functions. That's a solved problem.