Tente com coalesce função que retornará '0' se a
sum(orders.amount) é nulo para um mês específico: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
