Calculate the difference between sales of games from region ID 2 and region ID 3.
difference = SUBTRACT(SUM(num_sales WHERE region_id = 2), SUM(num_sales WHERE region_id = 3));
SELECT SUM(CASE WHEN T.region_id = 2 THEN T.num_sales ELSE 0 END) - SUM(CASE WHEN T.region_id = 3 THEN T.num_sales ELSE 0 END) FROM region_sales t