What is the average number of actors acted in comedy movies?
comedy is the name of a category; average number of actors refers to AVG(actor_id)
SELECT AVG(T1.actor_id) FROM film_actor AS T1 INNER JOIN film_category AS T2 ON T1.film_id = T2.film_id INNER JOIN category AS T3 ON T2.category_id = T3.category_id INNER JOIN actor AS T4 ON T4.actor_id = T1.actor_id WHERE T3.name = 'comedy'