How many units of item no.9 were sold in store no.1 in total in January, 2012?
store no. 1 refers to store_nbr = 1; item no. 9 refers to item_nbr = 9; in January refers to SUBSTR(date, 1, 4) = '2012' and SUBSTR(date, 6, 2) = '01'
SELECT SUM(units) FROM sales_in_weather WHERE SUBSTR(`date`, 6, 2) = '01' AND SUBSTR(`date`, 1, 4) = '2012' AND item_nbr = 9 AND store_nbr = 1