MySQL-使用子查询查找平均值

我对这个查询的语法有问题。我试图从每个国家返回一家公司的最大总数

这些表如下所示：

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

我尝试了以下方法：

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;

此查询仅返回三个国家/地区：

Ireland Hungry Owl All-Night Grocers    57317.3900
Norway  Sant Gourmet    5735.1500
Poland  Wolski  Zajazd  3531.9500

但是，我尝试的这个查询返回所有国家/地区，但我不确定它是否正确，因为我没有像在上一个查询中那样包含“max”值：

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;

我也不确定我是否正确计算了订单总数，这可能是我的错误。但我正试图从每个国家找到订单总量最大的公司。

---

对不起，所有的文字

以下查询将仅列出每个国家/地区订单总额最大的公司：

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
;

[更新]

请注意，上面的SQL遵循在列出的查询中计算

OrderSum

的方式。鉴于表

Products

包含

QuantityPerUnit

和

UnitPrice

，我怀疑您的

OrderSum

也应该乘以

QuantityPerUnit

——在这种情况下，您需要修改

OrderSum

的数学