What was the average temperature differences during May 2012 for store number 6 and 7?
during May 2012 refers to SUBSTR(date, 1, 7) = '2012-05'; store number 6 refers to store_nbr = 6; store number 7 refers to store_nbr = 7; average temperature difference = Subtract (Divide (Sum(tavg), Count (date) where the store_nbr = 6), Divide (Sum(tavg), Count(date) where store_nbr = 7))
SELECT ( SELECT CAST(SUM(tavg) AS REAL) / COUNT(`date`) FROM weather AS T1 INNER JOIN relation AS T2 ON T1.station_nbr = T2.station_nbr AND T1.`date` LIKE '%2012-05%' AND T2.store_nbr = 6 ) - ( SELECT CAST(SUM(tavg) AS REAL) / COUNT(`date`) FROM weather AS T1 INNER JOIN relation AS T2 ON T1.station_nbr = T2.station_nbr WHERE T1.`date` LIKE '%2012-05%' AND T2.store_nbr = 7 )