Correct sum with multiple subrecords (postgresql) - postgresql

Question

This have maybe been asked several times before, but I do not how to achieve a correct som from both parent and child.
Here is the tables:
CREATE TABLE co
(coid int4, coname text);
INSERT INTO co
(coid, coname)
VALUES
(1, 'Volvo'),
(2, 'Ford'),
(3, 'Jeep'),
(4, 'Toyota')
;
CREATE TABLE inv
(invid int4, invco int4, invsum numeric(10,2));
INSERT INTO inv
(invid, invco, invsum)
VALUES
(1,1,100),
(2,1,100),
(3,2,100),
(4,3,100),
(5,4,100)
;
CREATE TABLE po
(poid int4, poinv int4, posum int4);
INSERT INTO po
(poid, poinv, posum)
VALUES
(1,1,50),
(2,1,50),
(3,3,100),
(4,4,100)
;
I started with this simple query
SELECT coname, sum(invsum)
FROM inv
LEFT JOIN co ON coid=invco
GROUP BY 1
ORDER BY 1
Which gave a correct result:
coname sum
Ford 100
Jeep 100
Toyota 100
Volvo 200
Then I added the po record and the sums became incorrect:
SELECT coname, sum(posum) as po, sum(invsum)
FROM inv
LEFT JOIN co ON coid=invco
LEFT JOIN po ON poinv=invid
GROUP BY 1
ORDER BY 1
Which multiplied the sum for Volvo:
coname po sum
Ford 100 100
Jeep 100 100
Toyota (null) 100 (no records for po = correct)
Volvo 100 300 (wrong sum for inv)
How do I construct a query that gives correct result with multiple subrecords of po? (Window function?)
Sqlfiddle: http://sqlfiddle.com/#!15/0d90c/12

Answer 1

Do the aggregation before the joins. This is a little complicated in your case, because the relationship between co and po seems to require inv:
SELECT co.coname, p.posum, i.invsum
FROM co LEFT JOIN
(SELECT i.invco, sum(i.invsum) as invsum
FROM inv i
GROUP BY i.invco
) i
ON co.coid = i.invco LEFT JOIN
(SELECT i.invco, sum(po.posum) as posum
FROM po JOIN
inv i
ON po.poinv = i.invid
GROUP BY i.invco
) p
ON co.coid = p.invco
ORDER BY 1;
Note: I presume the logic is to keep everything in the co table, even if there are no matches in the other tables. The LEFT JOIN should start with this table, the one with all the rows you want to keep.