How many units are being sold for item 1 when the average temperature is 83?
item 1 refers to item_nbr = 1; when the average temperature is 83 refers to tavg = 83
SELECT SUM(units) FROM weather AS T1 INNER JOIN sales_in_weather AS T2 ON T1.`date` = T2.`date` INNER JOIN relation AS T3 ON T2.store_nbr = T3.store_nbr WHERE T2.item_nbr = 1 AND T1.tavg = 83