Access 2013 SQL, three tables, two using sum wrong results - sql

Question

Can someone please help me with this issue? I've scoured the Internet looking at dozens of examples, but i just can't find a solution that works.
I am using Access 2013. The problem is that I am trying to make a query that will highlight all part numbers from a supplier that either has customer back orders and/or overdue deliveries.
I am using three tables:
tbl_Inventory_Master which I require the part number, on hand stock value, and the supplier code.
For any back orders I need to join the tbl_Customer_Back_Order table as I need the count of back order lines and the sum of the back order quantity.
If the supplier has a late delivery, then I need to add the tbl_On_Order table showing the count of overdue deliveries and the sum of the overdue quantities.
The query is retrieving the data but the returned quantities are double what they should be.
SELECT
I.Inventory_Part_Num, I.Description, I.On_Hand_Stock,
COUNT (B.Part_Number) AS Back_Order_Count, SUM(B.Back_Order_Qty) as BO_Qty,
COUNT(O.Part_Number) AS Late_Deliveries_Count, SUM(O.Order_Qty) AS Late_Qty
FROM (tbl_Inventory_Master AS I
LEFT OUTER JOIN tbl_Customer_Back_Order AS B
ON I.Inventory_Part_Num = B.Part_Number)
LEFT OUTER tbl_On_Order AS O
ON I.Inventory_Part_Num = O.Part_Number
WHERE
I.Customer_Code = '274' AND
O.Due_Date < [ENTER TODAYS DATE IN FORMAT DD/MM/YYYY]
GROUP BY I.Inventory_Part_Num, I.Description, I.On_Hand_Stock
For example, for the part number 2022940 I should have 10 back order lines and an overdue quantity of 43. Instead, the query is returning 20 back order lines and an overdue quantity sum of 86.
From the on order table I have three orders totaling 144 pieces, instead the query is returning 960.
Can someone please advise, as this is driving me crazy?

Answer 1

You are joining along unrelated dimensions, so you need to aggregate before joining:
SELECT I.Inventory_Part_Num, I.Description, I.On_Hand_Stock,
B.Back_Order_Count, B.BO_Qty,
O.Late_Deliveries_Count, O.Late_Qty
FROM (tbl_Inventory_Master AS I LEFT OUTER JOIN
(SELECT B.Part_Number, COUNT(*) as Back_Order_Count,
SUM(B.Back_Order_Qty) as BO_Qty
FROM tbl_Customer_Back_Order AS B
GROUP BY B.Part_Number
) as B
ON I.Inventory_Part_Num = B.Part_Number
) LEFT JOIN
(SELECT O.Part_Number, COUNT(O.Part_Number) AS Late_Deliveries_Count,
SUM(O.Order_Qty) AS Late_Qty
FROM tbl_On_Order AS O
WHERE O.Due_Date < [ENTER TODAYS DATE IN FORMAT DD/MM/YYYY]
GROUP BY O.Part_Number
) as O
ON I.Inventory_Part_Num = O.Part_Number
WHERE I.Customer_Code = '274';
Notice the outer aggregation is no longer needed.