ORDER BY specifies how the rows are ordered in a partition, which determines the sort order of the rows over which the function is applied.
To add the monthly sales for a store in the sales_tbl table to the sales for previous months, compute the cumulative sales sum and order the rows in each partition by SMonth:
SELECT StoreID, SMonth, ProdID, Sales, SUM(Sales) OVER (PARTITION BY StoreID ORDER BY SMonth ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) FROM sales_tbl;
Result:
StoreID SMonth ProdID Sales Cumulative Sum(Sales) ------- ------ ------ --------- --------------------- 1001 1 C 35000.00 35000.00 1001 2 C 25000.00 60000.00 1001 3 C 40000.00 100000.00 1001 4 C 25000.00 125000.00 1001 5 C 30000.00 155000.00 1001 6 C 30000.00 185000.00 1002 1 C 40000.00 40000.00 1002 2 C 35000.00 75000.00 1002 3 C 110000.00 185000.00 1002 4 C 60000.00 245000.00 1002 5 C 35000.00 280000.00 1002 6 C 100000.00 380000.00