How many no.9 items from store no.11 were sold on 2012/12/7?
no. 9 item refers to item_nbr = 9; store no.11 refers to store_nbr = 11; sold on 2012/12/7 refers to date = '2012-12-07'
SELECT units FROM sales_in_weather WHERE `date` = '2012-12-07' AND store_nbr = 11 AND item_nbr = 9