Database: video_games
Question: Among the games released in 2004, what is the percentage of games on PSP?
SQL: SELECT CAST(COUNT(CASE WHEN T1.platform_name = 'PSP' THEN T3.game_id ELSE NULL END) AS REAL) * 100 / COUNT(T3.game_id) FROM platform AS T1 INNER JOIN game_platform AS T2 ON T1.id = T2.platform_id INNER JOIN game_publisher AS T3 ON T2.game_publisher_id = T3.id WHERE T2.release_year = 2004