php、postgresql、mysql

これを試してください

select  cont.name, cont.total,cp.contractor_id,cp.amount_paid,wt.id
                FROM
                (
                  select  name,id, sum(amount_to_be_paid) as total
                  from contractors group by name,id
                ) cont
                left join
                (
                  select contractor_id,sum(amount_paid) as amount_paid
                  from contractor_payments
                  group by contractor_id
                ) cp

                  on cont.id = cp.contractor_id
                              left join worktypeids as wt on cont.worktypeid_id = wt.id AND wt.project_id=2