Multi Table Query For A Summary Cost?

I'm wanting to create a query that will give me a summary of parts and labor from work orders. However there are three tables. work_order, parts, labor. Sometimes there will only be parts, sometimes just labor, or sometimes both. Well my query will only return results when they are on both. Sample query is below. Do I need to create a temp table to gather the data or is there a way to do this with a sub query.

select work_order.wono, sum(parts.cost), sum(labor.cost)
from work_order, parts, labor
where
work_order.wono = parts.wono and
work_order.wono = labor.wono
group by work_order.wono;