I'd recommend using a left join with table alias. Example
Select a.Project_PK , SUM(b.cost) AS Sum_of_Cost , SUM(b.estimate_cost) AS Sum_of_Estimated_Cost , SUM(c.quantity) AS Sum_of_Quantity , SUM(c.estimate_quantity) AS Sum_of_Estimated_Quantity FROM Projects a LEFT JOIN Cost b ON a.project_pk = b.project_pk LEFT JOIN Quantity c ON a.project_pk = c.project_pk -- Dixon Epperson
