What is the average number of horror movies among all movies genre?
horror movies refers to genre_name = 'horror'; average number = divide(sum(movie_id) when genre_name = 'horror', count(movie_id))
SELECT CAST(COUNT(CASE WHEN T3.genre_name = 'Horror' THEN T1.movie_id ELSE NULL END) AS REAL) / COUNT(T1.movie_id) FROM movie AS T1 INNER JOIN movie_genres AS T2 ON T1.movie_id = T2.movie_id INNER JOIN genre AS T3 ON T2.genre_id = T3.genre_id