For each product and month, compute running sum of 3 previous months' sales quantities, including the current month for the first 6 months (i.e., January through June). For example, for (cherry, 3), find sum of sales quantities from January, February and March.

For January, compute the sum for just 1 month (for January); and similarly, for February, compute the running sum of 2 months (for January and February).

sales (prod, cust, yr, mo, day, state, quant)

SHOULD PRINT OUT SOMETHING LIKE THIS (THIS IS NOT THE ANSWER, ONLY A SAMPLE)