Give the id of the bestsellers of store no.1 on 2012/1/1.
store no. 1 refers to store_nbr = 1; on 2012/1/1 refers to date = '2012-01-01'; best seller refers to Max(units); ID refers to item_nbr
SELECT item_nbr FROM sales_in_weather WHERE `date` = '2012-01-01' AND store_nbr = 1 ORDER BY units DESC LIMIT 1