Example: Default Window Frame for RANGE

SELECT prod_id, month_num, sales, SUM(sales) OVER
  (PARTITION BY prod_id ORDER BY month_num) 
FROM sale 
ORDER BY prod_id, month_num;

prod_id   month_num       sales    SUM(sales)
-------   ---------       -----    ----------
10                1         100           250
10                1         150           250
10                2         120           370
10                3         100           470
10                4         130           600
10                5         120           751
10                5          31           751
10                6         110           861
20                1          20            20
20                2          30            50
20                3          25            75
20                4          30           105
20                5          31           136
20                6          20           156
30                1          10            10
30                2          11            21
30                3          12            33
30                4           1            35
30                4           1            35

SELECT prod_id, month_num, sales, SUM(sales) OVER
  (PARTITION BY prod_id ORDER BY month_num RANGE
  BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) 
FROM sale 
ORDER BY prod_id, month_num;