法一：内连接+not in

select film_id as '电影id',title as '名称'
from film
where film_id not in(select f.film_id
from film f inner join film_category fc on f.film_id=fc.film_id)

思考了一下，还有更简单的
法二：左连接+is null

select f.film_id, f.title
from film f left join film_category fc on f.film_id=fc.film_id
where fc.category_id is null