select title,description from film f,film_category f_c
where f.film_id=f_c.film_id and category_id in
(select category_id from category where name ='action')
先用子查询从category中查出“action”分类的电影的category_id,再将film和film_category关联起来，查询category_id想对应的的电影名称和id