What is the item number of the product with the highest number of units sold in store number 1 on 1/1/2012?
item number refers to item_nbr; highest number of units sold refers to Max(units); store no.1 refers to store_nbr = 1; on 1/1/2012 refers to date = '2012-01-01'
SELECT item_nbr FROM sales_in_weather WHERE `date` = '2012-01-01' AND store_nbr = 1 ORDER BY units DESC LIMIT 1