How many units of item no.9 were sold in store no.1 on 2012/1/1?
store no. 1 refers to store_nbr = 1; item no. 9 refers to item_nbr = 9; on 2012/1/1 refers to date = '2012-01-01'
SELECT units FROM sales_in_weather WHERE `date` = '2012-01-01' AND store_nbr = 1 AND item_nbr = 9