1つの列でDISTINCTを実行し、TOP行を返します

最大の顧客3人をどのようにクエリしますPurchase_Costか？

DISTINCTonのみを適用したいのですCustomer_Nameが、以下のクエリは3つの列すべてに個別を適用します。クエリを変更して目的の出力を取得するにはどうすればよいですか？

SELECT DISTINCT TOP 3 customer_name, order_no, Purchase_Cost  
FROM PurchaseTable 
ORDER BY Purchase_Cost

次のクエリのdbnameとschemaNameを置き換えます。

;WITH CTE AS 
(
SELECT  
       [Order_No]
      ,[Customer_Name]
      ,[Purchase_Cost]
     , ROW_NUMBER() OVER(PARTITION BY [customer Name] ORDER BY [Purchase Cost] DESC) AS "RowNumber"
  FROM [dbname].[schemaName].[PurchaseTable]
  )

  SELECT TOP(3)
       [Order_No]
      ,[Customer_Name]
      ,[Purchase_Cost]
  FROM CTE WHERE RowNumber=1
  ORDER BY [Purchase_Cost] DESC

同じことをする他の方法があると私は確信しています。これを読むことをお勧めします。

してみてください：

SELECT DISTINCT TOP 3  order_no, customer_name,  Purchase_Cost
FROM
(   SELECT order_no, customer_name, Purchase_Cost, ROW_NUMBER() OVER(PARTITION BY customer_name ORDER BY Purchase_Cost DESC) Orders
    FROM PurchaseTable
) A
WHERE A.Orders = 1
ORDER BY Purchase_Cost DESC

（Customer_Name、Purchase_Cost DESC）INCLUDE（Order_No）のインデックスがあれば、これは他のソリューションよりも効率的かもしれません。

;
WITH PurchaseTable AS
(
    SELECT * 
      FROM (VALUES ((501),('Carson'),(3400)),
                   ((502),('Thomas'),(625)),
                   ((503),('Daisy'),(4856)),
                   ((504),('Mary'),(2397)),
                   ((505),('Carson'),(5000))
           ) AS T(Order_No,Customer_Name,Purchase_Cost)
),
DistinctCustomers AS
(
    SELECT DISTINCT
           Customer_Name
      FROM PurchaseTable
)
SELECT TOP(3) 
       MaxCustomerOrder.Order_No,
       dc.Customer_Name,
       MaxCustomerOrder.Purchase_Cost
  FROM DistinctCustomers dc
 CROSS APPLY (SELECT TOP(1) * 
                FROM PurchaseTable pt 
               WHERE pt.Customer_Name = dc.Customer_Name 
               ORDER BY pt.Purchase_Cost DESC
             )    AS MaxCustomerOrder
 ORDER BY MaxCustomerOrder.Purchase_Cost DESC
;

またはインラインテーブル定義なし：

;
WITH DistinctCustomers AS
(
    SELECT DISTINCT
           Customer_Name
      FROM PurchaseTable
)
SELECT TOP(3) 
       MaxCustomerOrder.Order_No,
       dc.Customer_Name,
       MaxCustomerOrder.Purchase_Cost
  FROM DistinctCustomers dc
 CROSS APPLY (SELECT TOP(1) * 
                FROM PurchaseTable pt 
               WHERE pt.Customer_Name = dc.Customer_Name 
               ORDER BY pt.Purchase_Cost DESC
             )    AS MaxCustomerOrder
 ORDER BY MaxCustomerOrder.Purchase_Cost DESC
;