-- Las funciones de agregado tienen sentido cuando agrupamos
-- los valores: GROUP BY
-- La sintaxis es select valor, agregado(..) from tabla1 join tabla2 GROUP BY valor
-- Cuantas ciudades tiene cada pais
select country,count(city) ciudades
from country join city on country.country_id=city.country_id
group by country;
-- Total de pagos por cliente
select first_name,last_name,sum(amount) total, avg(amount) media, min(amount) minimo,max(amount) maximo
from customer join payment on customer.customer_id=payment.customer_id
group by customer.customer_id
order by first_name,last_name;
-- Total de películas por categoría
select name,count(film_id) total from category
join film_category on category.category_id=film_category.category_id
group by category.category_id
order by total desc;
