先找出有分类的电影id， 然后用排除法去掉

with film_in as
(select f.film_id 
from film f 
join film_category fc 
on f.film_id=fc.film_id
join category c 
on fc.category_id=c.category_id)
select film_id, title
from film
where film_id not in 
(select film_id from film_in)