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