What is the average ratio between female and male actors in a movie?
female refers to gender = 'Female'；male refers to gender = 'Male'; average ratio = divide(divide(sum(gender_id) when gender = 'Female', sum(gender_id) when gender = 'Male'), count(movie_id)) as percentage
SELECT CAST(COUNT(CASE WHEN T2.gender = 'Female' THEN T1.person_id ELSE NULL END) AS REAL) / COUNT(CASE WHEN T2.gender = 'Male' THEN T1.person_id ELSE NULL END) FROM movie_cast AS T1 INNER JOIN gender AS T2 ON T1.gender_id = T2.gender_id