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