How many units of item no.5 were sold in store no.3 on the day in 2012 when the max temperature was the highest?
item no.5 refers to item_nbr = 5; store no. 3 refers to store_nbr = 3; when the max temperature was highest refers to Max(tmax); in 2012 refers to SUBSTR(date, 1, 4) = '2012'
SELECT T1.units 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 ORDER BY tmax DESC LIMIT 1