@Royi:na verdade, acho que deveria ser
select customers.customerId , count(orders.id) as num
from customers join orders on custumers.Id = orders.customerId
group by customers.customerId
having num=2
select customers.customerId , count(orders.id) as num
from customers join orders on custumers.Id = orders.customerId
group by customers.customerId
having num=2
