Sqlite substract sums (with group by) with JOIN and duplicates - sql

Question

I previously found the solution to my problem but unfortunately I lost files on my harddrive and I can't find the statement I managed to produce.
I have 2 tables T2REQ and T2STOCK, both have 2 columns (typeID and quantity) and my problem reside in the fact that I can have multiple occurences of SAME typeID in BOTH tables.
What I'm trying to do is SUM(QUANTITY) grouped by typeID and substract the values of T2STOCK from T2REQ but since I have multiple occurences of same typeID in both tables, the SUM I get is multiplied by the number of occurences of typeID.
Here's a sample of T2REQ (take typeID 11399 for example):
typeID quantity
---------- ----------
34 102900
35 10500
36 3220
37 840
11399 700
563 140
9848 140
11486 28
11688 700
11399 390
4393 130
9840 390
9842 390
11399 390
11483 19.5
11541 780
And this is a sample of T2STOCK table :
typeID quantity
---------- ----------
9842 1921
9848 2400
11399 1700
11475 165
11476 27
11478 28
11481 34
11483 122
11476 2
And this is where I'm at for now, I know that the SUM(t2stock.quantity) is affected (multiplied) because of the JOIN 1 = 1 but whatever I tried, I'm not doing it in the right order:
SELECT
t2req.typeID, sum(t2req.quantity), sum(t2stock.quantity),
sum(t2req.quantity) - sum(t2stock.quantity) as diff
FROM t2req JOIN t2stock ON t2req.typeID = t2stock.typeID
GROUP BY t2req.typeID
ORDER BY diff DESC;
typeID sum(t2req.quantity) sum(t2stock.quantity) diff
---------- ------------------- --------------------- ----------
563 140 30 110
11541 780 780 0
11486 28 40 -12
11483 19.5 122 -102.5
9840 390 1000 -610
40 260 940 -680
9842 390 1921 -1531
9848 140 2400 -2260
11399 1480 5100 -3620
39 650 7650 -7000
37 1230 116336 -115106
36 28570 967098 -938528
35 33770 2477820 -2444050
34 102900 2798355 -2695455
You can see that SUM(t2req) for typeID 11399 is correct : 1480
And you can see that the SUM(t2stock) for typeID 11399 is not correct : 5100 instead of 1700 (which is 5100 divided by 3, the number of occurences in t2req)
What would be the best way to avoid multiplications because of multiple typeIDs (in both tables) with the JOIN for my sum substract ?
Sorry for the wall of text, just trying to explain as best as I can since english is not my mother tongue.
Thanks a lot for your help.

Answer 1

You can aggregate before join:
SELECT
t2req.typeID,
t2req.quantity,
t2stock.quantity,
t2req.quantity - t2stock.quantity as diff
FROM
(SELECT TypeID, SUM(Quantity) Quantity FROM t2req GROUP BY TypeID) t2req JOIN
(SELECT TypeID, SUM(Quantity) Quantity FROM t2stock GROUP BY TypeID) t2stock
ON t2req.typeID = t2stock.typeID
ORDER BY diff DESC;
Fiddle sample: http://sqlfiddle.com/#!7/06711/5

Answer 2

You can't do this in a single aggregation:
SELECT
COALESCE(r.typeID, s.typeID) AS typeID,
COALESCE(r.quantity, 0) AS req_quantity,
COALESCE(s.quantity, 0) AS stock_quantity,
COALESCE(r.quantity, 0) - COALESCE(s.quantity, 0) AS diff
FROM (
SELECT rr.typeID, SUM(rr.quantity) AS quantity
FROM t2req rr
GROUP BY rr.typeID
) r
CROSS JOIN (
SELECT ss.typeID, SUM(ss.quantity) AS quantity
FROM t2stock ss
GROUP BY ss.typeID
) s ON r.typeID = s.typeID
ORDER BY 4 DESC;