SQL JOIN omits some fields - sql

Question

I have the following tables:
Product_T with columns:
ProductID,
ProductDescription
OrderLine_T with columns:
OrderID,
ProductID,
OrderedQuantity
Order_T with columns:
OrderID,
CustomerID,
Customer_T with columns:
CustomerID,
CustomerName
I want to list the product ID and description, along with the customer ID and name for the customer who has bought the most of that product and also show the total quantity ordered by that customer.
I came up with following query, to list the max quantity product per order:
SELECT o1.OrderID, o1.ProductID, SUM(o1.OrderedQuantity) AS A
FROM OrderLine_T o1
GROUP BY
o1.ProductID,
o1.OrderID
HAVING SUM(o1.OrderedQuantity) = (
SELECT MAX(s.d)
FROM (
SELECT
o1.OrderID,
o1.ProductID,
SUM(o1.OrderedQuantity) AS d
FROM OrderLine_T o1
GROUP BY
o1.ProductID,
o1.OrderID
) s
WHERE o1.ProductID = s.ProductID
)
And that gave me a correct output of:
50 20 1
48 17 5
32 14 10
59 13 2
1 10 9
2 8 2
69 7 4
4 6 3
32 5 10
55 4 2
2 3 12
1 2 18
26 1 5
But then, when I tried joining it with other tables, so I could select CustomerName and CustomerID, like so:
SELECT
o1.ProductID,
s.CustomerName,
s.CustomerID,
SUM(o1.OrderedQuantity) AS A
FROM OrderLine_T o1
INNER JOIN (
SELECT
c1.CustomerName,
c1.CustomerID,
p1.ProductID
FROM Product_T p1
INNER JOIN OrderLine_T o3 ON p1.ProductID = o3.ProductID
INNER JOIN Order_T o2 ON o3.OrderID = o2.OrderID
INNER JOIN Customer_T c1 ON o2.CustomerID = c1.CustomerID
) s ON s.ProductID = o1.ProductID
GROUP BY
o1.ProductID,
s.CustomerName,
s.CustomerID
HAVING SUM(o1.OrderedQuantity) = (
SELECT MAX(s.d)
FROM (
SELECT
o1.OrderID,
o1.ProductID,
SUM(o1.OrderedQuantity) AS d
FROM OrderLine_T o1
GROUP BY
o1.ProductID,
o1.OrderID
) s
WHERE o1.ProductID = s.ProductID
) ;
The output shrunk to:
17 Contemporary Casuals 1 5
8 Home Furnishings 3 2
7 Eastern Furniture 4 4
10 Eastern Furniture 4 9
20 Dunkins Furniture 8 1
13 Ikards 13 2
Why could that be?

Answer 1

It seems you should be using window functions here, such as ROW_NUMBER, along with conditional aggregation
SELECT
o.ProductID,
p.Description,
CustomerID = MAX(CASE WHEN o.rn = 1 THEN c.CustomerID END),
CustomerName = MAX(CASE WHEN o.rn = 1 THEN c.CustomerName END),
SUM(CASE WHEN o.rn = 1 THEN o.TotalQty END) AS QtyForTopCustomer
SUM(o.TotalQty) AS TotalQty
FROM (
SELECT
o.ProductID,
o.CustomerID,
TotalQty = SUM(oi.OrderedQuantity),
rn = ROW_NUMBER() OVER (PARTITION BY oi.ProductId ORDER BY SUM(oi.OrderedQuantity) DESC)
FROM OrderLine_T ol
INNER JOIN Order_T o ON o.OrderID = ol.OrderID
GROUP BY
o.ProductID,
o.CustomerID
) o
INNER JOIN Customer_T c ON c.CustomerID = o.CustomerID
INNER JOIN Product_T p ON p.ProductID = ol.ProductID
GROUP BY
o.ProductID,
p.Description;
If you only wanted the data for that one customer, you could remove the conditional aggregation and just filter by row-number
SELECT
o.ProductID,
p.Description,
o.CustomerID,
o.CustomerName,
o.TotalQty
FROM (
SELECT
p.ProductID,
p.Description,
o.CustomerID,
TotalQty = SUM(oi.OrderedQuantity),
rn = ROW_NUMBER() OVER (PARTITION BY oi.ProductId ORDER BY SUM(oi.OrderedQuantity) DESC)
FROM OrderLine_T ol
INNER JOIN Order_T o ON o.OrderID = ol.OrderID
GROUP BY
p.ProductID,
p.Description,
o.CustomerID
) o
INNER JOIN Customer_T c ON c.CustomerID = o.CustomerID
INNER JOIN Product_T p ON p.ProductID = ol.ProductID
WHERE o.rn = 1;