在SQL Server 2012中查找最頻繁的值

Question

我想找到每個客戶購買的最頻繁的產品。 我的數據集是這樣的：

CustomerID     ProdID    FavouriteProduct
    1              A              ?
    1              A              ?
    1              A              ?
    1              B              ?
    1              A              ?
    1              A              ?
    1              A              ?
    1              B              ?
    2              A              ?
    2              AN             ?
    2              G              ?
    2              C              ?
    2              C              ?
    2              F              ?
    2              D              ?
    2              C              ?

產品太多了，所以我不能將它們放在數據透視表中。

答案看起來像這樣：

CustomerID     ProdID    FavouriteProduct
    1              A              A
    1              A              A
    1              A              A
    1              B              A
    1              A              A
    1              A              A
    1              A              A
    1              B              A
    2              A              C
    2              AN             C
    2              G              C
    2              C              C
    2              C              C
    2              F              C
    2              D              C
    2              C              C

該查詢可能看起來像這樣：

Update table
set FavouriteProduct = (Select 
                            CustomerID, Product, Max(Count(Product)) 
                        From Table 
                        group  by CustomerID, Product) FP    

Answer 1

獲得最頻繁產品的另一種方法是使用row_number() ：

select customerid, productid,
       max(case when seqnum = 1 then productid end) over (partition by customerid) as favoriteproductid
from (select customerid, productid, count(*) as cnt,
             row_number() over (partition by customerid order by count(*) desc) as seqnum
      from customer c
      group by customerid, productid
     ) cp;

Answer 2

要完全按照問題中的描述返回行，可以嘗試使用表表達式（在示例中使用CTE）首先返回受歡迎程度排名，其中數字越大，對於每個客戶而言產品越受歡迎。

WITH RankTable AS (
  SELECT
    CustomerID, ProductID, COUNT(*) AS Popularity
  FROM TableA
  GROUP BY CustomerID, ProductID
)

然后，可以通過首先在原始表（TableA）和表表達式（RankTable）上執行內部聯接，然后使用窗口函數在FavoriteProduct列中創建值來返回完整結果表。

SELECT 
    P.CustomerID
  , P.ProductID
  , FIRST_VALUE(P.ProductID) OVER(
      PARTITION BY R.CustomerID
      ORDER BY R.Popularity DESC, R.ProductID) AS FavoriteProduct
FROM TableA AS P
  INNER JOIN RankTable AS R
    ON P.CustomerID = R.CustomerID
    AND P.ProductID= R.ProductID;

Answer 3

多虧了尼克，我找到了一種找到最頻繁的價值的方法。 我與您分享它是如何工作的：

   Select CustomerID,ProductID,Count(*) as Number 
   from table A 
   group by CustomerID,ProductID 
   having Count(*)>= (Select Max(Number) from (Select CustomerID,ProductID,Count(*) as Number from table B where B.CustomerID= A.CustomerID  group by CustomerID,Product)C) 

Answer 4

萬一您的SQL執行得不夠快，並且您的客戶也位於較小的表中，這可能會更好：

select C.CustomerId, R.ProductID
from Customer C
outer apply (
  Select top 1 ProductID,Count(*) as Number 
  from table A 
  where A.CustomerId = C.CustomerId
  group by ProductId
  order by Number desc 
) R

Answer 5

基於本頁面末尾的示例，這可能更快一些： http ://www.sql-server-performance.com/2006/find-frequent-values/：

SELECT CustomerID, ProdID, Cnt 
FROM 
(
    SELECT CustomerID, ProdID, COUNT(*) as Cnt, 
    RANK() OVER (
       PARTITION BY CustomerID
       ORDER BY COUNT(*) DESC
    ) AS Rnk 
    FROM YourTransactionTable
    GROUP BY CustomerID, ProdID
) x 
WHERE Rnk = 1

這個使用了RANK()函數。 在這種情況下，您不必重新連接到同一張表（這意味着需要做的工作要少得多）

現在要更新您的現有數據，我想將我的數據集包裝在WITH中，以使調試更容易，並且最終更新也更簡單：

;WITH
(
  SELECT CustomerID, ProdID, Cnt 
  FROM 
  (
     SELECT CustomerID, ProdID, COUNT(*) as Cnt, 
     RANK() OVER (PARTITION BY CustomerID
     ORDER BY COUNT(*) DESC) AS Rnk 
     FROM TransactionTable
     GROUP BY CustomerID, ProdID
  ) x 
  WHERE Rnk = 1
) As SRC

 UPDATE FavouriteTable
 SET Favourite = SRC.ProdID
 FROM SRC
 WHERE SRC.CustomerID = Favourite.CustomerID