Опитайте с coalesce
функция, която ще върне '0', ако sum(orders.amount)
е нулев за конкретен месец:
select COALESCE(sum(o.amount), 0) as amount, o.completed_at
from orders o
where date(o.completed_at) between '2011/10/01' and '2012/03/06'
group by year(o.completed_at), month(o.completed_at) order by o.completed_at