What was the difference of number of units sold in station number 1 and number 2 on year 2012?
station 1 refers to station_nbr = 1; station 2 refers to station_nbr = 2; on year 2012 refers to substring (date, 1, 4) = '2012'; difference = Subtract (Sum(units where station_nbr = 1), Sum(units where station_nbr = 2))
SELECT SUM(CASE WHEN T1.station_nbr = 1 THEN units ELSE 0 END) - SUM(CASE WHEN T1.station_nbr = 2 THEN units ELSE 0 END) FROM relation AS T1 INNER JOIN sales_in_weather AS T2 ON T1.store_nbr = T2.store_nbr WHERE T2.`date` LIKE '%2012%'