MySQL - Using subquery to find an average - mysql

Question

I am having trouble with the syntax of this query. I am trying to return the largest total for one company from each country.
The tables look like:
Orders
Columns
OrderID
CustomerID
EmployeeID
OrderDate
RequiredDate
OrderDetails
Columns
OrderID
ProductID
UnitPrice
Quantity
Products
Columns
ProductID
ProductName
QuantityPerUnit
UnitPrice
Customers
Columns
CustomerID
CompanyName
ContactName
Country
I have tried the following:
SELECT
T1.Country,
CompanyName,
T1.OrderSum
FROM
(SELECT
C.Country,
C.CompanyName,
SUM(UnitPrice * Quantity) AS OrderSum
FROM Customers C
JOIN Orders O
ON C.CustomerID = O.CustomerID
JOIN OrderDetails D
ON D.OrderID = O.OrderID
GROUP BY C.Country) T1
JOIN
-- TOP PAYMENT TOTALS BY COUNTRY
(SELECT COUNTRY,
MAX(OrderSum) AS OrderSum
FROM
-- PAYMENT TOTALS BY CUSTOMER
(SELECT C.Country,
C.CompanyName,
SUM(UnitPrice * Quantity) AS OrderSum
FROM Customers C
JOIN Orders O1
ON O1.CustomerID = C.CustomerID
JOIN OrderDetails D1
ON D1.OrderID = O1.OrderID
GROUP BY C.COUNTRY, C.CompanyName) T2
GROUP BY COUNTRY) T3
ON T1.COUNTRY = T3.COUNTRY
AND T1.OrderSum = T3.OrderSum
ORDER BY Country;
This query only returns three countries:
Ireland Hungry Owl All-Night Grocers 57317.3900
Norway Sant Gourmet 5735.1500
Poland Wolski Zajazd 3531.9500
But, this query I tried, returns all countries, but I am not sure if it is correct because I did not include the 'max' value as I did in the previous query:
SELECT
T1.Country,
CompanyName,
T1.OrderSum
FROM
(SELECT
C.Country,
C.CompanyName,
SUM(UnitPrice * Quantity) AS OrderSum
FROM Customers C
JOIN Orders O
ON C.CustomerID = O.CustomerID
JOIN OrderDetails D
ON D.OrderID = O.OrderID
GROUP BY C.CompanyName) T1
GROUP By Country
ORDER BY Country;
I am also unsure if I am calculating the order totals correctly, which may be the mistake on my part. But I am trying to find the company from each country that has the largest order total.
Sorry for all the text.

Answer 1

The following query would list only the companies with the largest order total per country:
SELECT A.Country, A.CompanyName, A.OrderSum
FROM (
SELECT
C.Country,
C.CompanyName,
SUM(D.UnitPrice * D.Quantity) AS OrderSum
FROM Customers C
JOIN Orders O ON C.CustomerID = O.CustomerID
JOIN OrderDetails D ON D.OrderID = O.OrderID
GROUP BY
C.Country, C.CustomerID
) A
JOIN (
SELECT
S.Country, MAX(S.OrderSum) as MaxSum
FROM (
SELECT
C.Country,
C.CompanyName,
SUM(D.UnitPrice * D.Quantity) AS OrderSum
FROM Customers C
JOIN Orders O ON C.CustomerID = O.CustomerID
JOIN OrderDetails D ON D.OrderID = O.OrderID
GROUP BY
C.Country, C.CustomerID
) S
GROUP BY
S.Country
) B ON A.Country = B.Country
WHERE
A.Country = B.Country AND
A.OrderSum = B.MaxSum
ORDER BY
A.Country, A.CompanyName
;
[UPDATE]
Note that the above SQL follows the way OrderSum is calculated in your listed queries. Given that table Products has QuantityPerUnit and UnitPrice, I suspect that your OrderSum should be multiplied by QuantityPerUnit as well – in which case you'll need to revise the math for OrderSum.