показать клиенту, у которого была самая большая сумма заказа в базе данных Northwind

Question

Я использую базу данных Northwind, и для каждого региона мне нужно показать клиенту, у которого был самый высокий объем заказа

Мои таблицы:

-- Customers --   
[CustomerID] [nchar](5) NOT NULL,    
[CompanyName] [nvarchar](40) NOT NULL,   
[ContactName] [nvarchar](30) NULL,    
[ContactTitle] [nvarchar](30) NULL,    
[Address] [nvarchar](60) NULL,  
[City] [nvarchar](15) NULL,    
[Region] [nvarchar](15) NULL,    
[PostalCode] [nvarchar](10) NULL,   
[Country] [nvarchar](15) NULL,   
[Phone] [nvarchar](24) NULL,    
[Fax] [nvarchar](24) NULL,

--OrderDetails
[OrderID] [int] NOT NULL,
[ProductID] [int] NOT NULL,
[UnitPrice] [money] NOT NULL,
[Quantity] [smallint] NOT NULL,
[Discount] [real] NOT NULL,
CONSTRAINT [PK_Order_Details] PRIMARY KEY CLUSTERED

-- Orders
[OrderID] [int] IDENTITY(1,1) NOT NULL,
[CustomerID] [nchar](5) NOT NULL,
[EmployeeID] [int] NULL,
[OrderDate] [datetime] NULL,
[RequiredDate] [datetime] NULL,
[ShippedDate] [datetime] NULL,
[ShipVia] [int] NULL,
[Freight] [money] NULL,
[ShipName] [nvarchar](40) NULL,
[ShipAddress] [nvarchar](60) NULL,
[ShipCity] [nvarchar](15) NULL,
[ShipRegion] [nvarchar](15) NULL,  
[ShipPostalCode] [nvarchar](10) NULL,
[ShipCountry] [nvarchar](15) NULL,
CONSTRAINT [PK_Orders] PRIMARY KEY CLUSTERED   

Я начинаю спытаюсь подсчитать для каждого клиента его общую сумму

select 
    sum(unitprice * quantity * 1+Discount) as ValueOfOrders
from 
    OrderDetails od 
join 
    orders o on od.OrderID = o.OrderID 
group by 
    CustomerID

Но я не знаю, как использовать функцию max, чтобы связать ее с регионом.

Answer 1

Ленивый способ сделать это

;WITH totals AS
(
select  c.Region, o.CustomerID, 
        SUM(unitprice * quantity * 1+Discount) AS Total
from [Order Details] od 
join orders o on od.OrderID = o.OrderID 
join Customers c on c.CustomerID = o.CustomerID
group by c.Region, o.CustomerID
)
, byRegion AS
(
select *, 
    ROW_NUMBER() OVER(PARTITION BY Region ORDER BY Total DESC) as rn 
from totals

)

select *
from byRegion 
where rn = 1
order by Region

Answer 2

Как насчет

select TOP 1
    sum(unitprice * quantity * (1+Discount)) as ValueOfOrders
from 
    OrderDetails od 
join 
    orders o on od.OrderID = o.OrderID 
group by 
    CustomerID
ORDER BY ValueOfOrders DESC;

Или даже с помощью CTE или подзапроса, чтобы получить MAX()

SELECT MAX(ValueOfOrders) ValueOfOrders
FROM
(
select 
    sum(unitprice * quantity * 1+Discount) as ValueOfOrders
from 
    OrderDetails od 
join 
    orders o on od.OrderID = o.OrderID 
group by 
    CustomerID
) T