The following SQL query might yield the results that follow it, where the cumulative maximum value for sales is returned for each partition defined by city and kind.
SELECT city, kind, sales, week, MAX(sales) OVER (PARTITION BY city, kind ORDER BY week ROWS UNBOUNDED PRECEDING) FROM activity_month;
Result:
city kind sales week Cumulative Max(sales) ------- -------- ----- ---- --------------------- LA Canvas 263 16 263 LA Canvas 294 17 294 LA Canvas 321 18 321 LA Canvas 274 20 321 LA Leather 144 16 144 LA Leather 826 17 826 LA Leather 489 20 826 LA Leather 555 21 826 Seattle Canvas 100 16 100 Seattle Canvas 182 17 182 Seattle Canvas 94 18 182 Seattle Leather 933 16 933 Seattle Leather 840 17 933 Seattle Leather 899 18 933 Seattle Leather 915 19 933 Seattle Leather 462 20 933