Calculate the difference in sales between the games released in 1990 and 2000.
difference = SUBTRACT(SUM(num_sales WHERE release_year = 2000), SUM(num_sales WHERE release_year = 1990));
SELECT SUM(CASE WHEN T2.release_year = 2000 THEN T1.num_sales ELSE 0 END) - SUM(CASE WHEN T2.release_year = 1990 THEN T1.num_sales ELSE 0 END) FROM region_sales AS T1 INNER JOIN game_platform AS T2 ON T1.game_platform_id = T2.id