How many items were sold by store 9 during a snowy day?
store 9 refers to store_nbr = 9; snowy day refers to snowfall < > 0 and snowfall is not null; item refers to item_nbr
SELECT COUNT(DISTINCT item_nbr) FROM weather AS T1 INNER JOIN relation AS T2 ON T1.station_nbr = T2.station_nbr INNER JOIN sales_in_weather AS T3 ON T2.store_nbr = T3.store_nbr WHERE T3.store_nbr = 9 AND T1.snowfall <> 0 AND T1.snowfall IS NOT NULL