SQL multiple 1-to-many joins - sql

Question

I'm almost certain I've run into this before and am just having an extended senior moment, but I am trying to pull work order data from three different tables across 2 db's on a SQL instance and combine it all into a report, I'm looking for the end result to contain the following columns:
WO | Production Recorded Qty | Inventory Qty | Variance
The Variance part is easy I can just nest the select statement, and subtract the two quantities in the outer statement, but the problem I'm running in to is when I join the production and Inventory tables in their corresponding databases I end up getting sums of the columns that I'm targeting that are way larger than what they should be
Sample Data:
Work Order, Short P/N, and Long P/N in Work Order Table:
dba.1
WO ShortPN LongPN
152 1234 Part1
Short P/N, Quantity on hand, location, and lot # in inventory table:
dba.2
ShortPN Qty Loc Lot
1234 31 Loc1 456
1234 0 Loc2 456
1234 0 Loc4 456
1234 19 Loc1 789
1234 25 Loc4 789
Work Order, Long P/N, and production count of the last 5min in Production table:
dbb.3
WO LongPN Count
152 Part3 6
152 Part3 8
152 Part3 9
152 Part3 4
152 Part3 6
152 Part3 7
With this example I've tried:
SELECT 1.WO AS WO
,SUM(2.Qty) AS "Qty On Hand"
,SUM(3.Count) AS "Produced Qty"
FROM dba.1
INNER JOIN dbb.2 ON 1.ShortPN=2.ShortPN
INNER JOIN dbb.3 ON 1.WO = 3.WO
GROUP BY 1.WO
I've also tried selecting from 3, joining 1 to 3 on the WO, then 2 to 1 on shortPN, and both yield SUM() numbers that are exponentially higher than they should be(ie what should be 15,xxx turns into over 2,000,000), however if I remove one of the data points from the report and select just the inventory or production qty I get the correct end results. I swear that I've run into this before but for the life of me can't remember how it was solved, sorry if it's a duplicate question as well, couldn't find anything by searching.
Thanks in advance for the help, it's greatly appreciated.

Answer 1

You can do something like this
select
WO.WO, isnull(i.Qty, 0) as Qty, isnull(p.[Count], 0) as [Count]
from WorkOrder as WO
left outer join (select t.ShortPN, sum(t.Qty) as Qty from inventory as t group by t.ShortPN) as i on
i.ShortPN = WO.ShortPN
left outer join (select t.WO, sum(t.[Count]) as [Count] from Production as t group by t.WO) as p on
p.WO = WO.WO
SQL FIDDLE example
if you have SQL Server 2005 or higher, you can write it like this
select
WO.WO, isnull(i.Qty, 0) as Qty, isnull(p.[Count], 0) as [Count]
from WorkOrder as WO
outer apply (select sum(t.Qty) as Qty from inventory as t where t.ShortPN = WO.ShortPN) as i
outer apply (select sum(t.[Count]) as [Count] from Production as t where t.WO = WO.WO) as p
SQL FIDDLE example
this happens because when you make a join of WO and inventory tables you got
WO SHORTPN QTY
-------------------
152 1234 31
152 1234 0
152 1234 0
152 1234 19
152 1234 25
and you see that now you have 5 rows with WO = 152. When you add join with Production table, for each row with WO = 152 from this join there will be 6 rows with WO = 152 from Production table, so you will have 30 rows total and QTY from inventory will be listed 6 times each. When you sum this up, instead of 75 you will have 75 * 6 = 450. And for Count you'll have each Count * 5, so instead of 40 you'll have 200.