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
database name:db_id video_games
genre table genre Cols: id dtype integer, genre_name dtype text, 
game table game Cols: id dtype integer, genre id dtype integer, game name dtype text, 
platform table platform Cols: id dtype integer, platform_name dtype text, 
publisher table publisher Cols: id dtype integer, publisher_name dtype text, 
game_publisher table game_publisher Cols: id dtype integer, game id dtype integer, publisher id dtype integer, 
game_platform table game_platform Cols: id dtype integer, game publisher id dtype integer, platform id dtype integer, release year dtype integer, 
region table region Cols: id dtype integer, region_name dtype text, 
region_sales table region_sales Cols: region_id dtype integer, game platform id dtype integer, number sales dtype real, 
