Question: What was the difference of number of units sold in station number 1 and number 2 on year 2012?
SQL: 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%'