Unions..or maybe not?

Question

I have a table named 'PROJTASK' which represents a task. Each task can have no, one or multiple 'PROJMTRL' and 'PROJLABR' which represent materials and time for the task. Each PROJMTRL and PROJLABR have a 'COST' variable.
NOW:
I'm trying to build a view, which lists all PROJTASKs, with the summarized COSTs of all PROJMTRL and PROJLABR.
(Actually there are 4 tables, but to understand the query, I start with 2 only..)
I don't quite get how unions work, I guess I need them in this case!??
Thanks for any help!!

Answer 1

create table proj (objectid number);
create table matl (objectid number, cost number);
create table labr (objectid number, cost number);
insert into proj values (1);
insert into matl values (1, 10);
insert into matl values (1, 20);
insert into matl values (1, 30);
insert into matl values (1, 40);
insert into labr values (1, 10);
insert into labr values (1, 20);
insert into labr values (1, 30);
select objectid, max(mcost), max(lcost)
from (
     select t.objectid, sum(m.cost) as mcost, 0 as lcost
     from proj t, matl m
     where t.objectid = m.objectid (+)
     group by t.objectid
     union
     select t.objectid, 0, sum(l.cost)
     from proj t, labr l
     where t.objectid = l.objectid (+)
     group by t.objectid
group by objectid;
  OBJECTID MAX(MCOST) MAX(LCOST)
         1        100         60
SQL>