What is the total number of units of item no.5 sold in store no.3 in 2012 on days when the temperature was below the 30-year normal?
item no.5 refers to item_nbr = 5; store no. 3 refers to store_nbr = 3; when the temperature was below the 30-year normal refers to depart < 0; in 2012 refers to SUBSTR(date, 1, 4) = '2012'; total number of units refers to Sum(units)
SELECT SUM(CASE WHEN T3.depart < 0 THEN units ELSE 0 END) AS sum FROM sales_in_weather AS T1 INNER JOIN relation AS T2 ON T1.store_nbr = T2.store_nbr INNER JOIN weather AS T3 ON T2.station_nbr = T3.station_nbr WHERE T2.store_nbr = 3 AND SUBSTR(T1.`date`, 1, 4) = '2012' AND T1.item_nbr = 5