4.
select avg(amount) from payment

5.
select count(film_category.film_id), category.name from film_category left join category on film_category.category_id = category.category_id group by category.name order by name asc

6.
select customer.first_name, customer.last_name, country.country from customer inner join address on customer.address_id=address.address_id inner join city on address.city_id=city.city_id inner join country on city.country_id=country.country_id order by country.country asc