What was the average temperature difference between store number 18 and 19 on 16 September 2022?
store number 18 refers to store_nbr = 18; store number 19 refers to store_nbr = 19; on 16 September 2022 refers to date = '2022-09-16'; average temperature difference = Subtract(tavg where store_nbr = 18, tavg where store_nbr = 19)
SELECT SUM(CASE WHEN T1.store_nbr = 18 THEN T2.tavg ELSE 0 END) - SUM(CASE WHEN T1.store_nbr = 19 THEN T2.tavg ELSE 0 END) FROM relation AS T1 INNER JOIN weather AS T2 ON T1.station_nbr = T2.station_nbr WHERE T2.`date` = '2012-09-16'