在嵌套選擇分區依據返回奇怪結果

Question

我遇到了SQLServer 2008中一個奇怪的行爲，我不明白。我想快速將一位獨特的客戶與獨特的付款配對。

使用此查詢，我得到我期待的結果。每個CustomerId與另一個PaymentId配對。

SELECT CustomerId, PaymentId, RowNumber1, RowNumber2 
FROM (
    SELECT 
     c.Id as CustomerId, 
     p.Id as PaymentId, 
     ROW_NUMBER() OVER (PARTITION BY p.Id ORDER BY p.Id) AS RowNumber1, 
     ROW_NUMBER() OVER (PARTITION BY c.Id ORDER BY c.Id) AS RowNumber2 
    FROM Customer as c 
    CROSS JOIN Payment as p 
) AS INNERSELECT WHERE RowNumber2 = 1 

+------------+-----------+------------+------------+ 
| CustomerId | PaymentId | RowNumber1 | RowNumber2 | 
+------------+-----------+------------+------------+ 
|   4 |   1 |   1 |   1 | 
|   5 |   2 |   2 |   1 | 
+------------+-----------+------------+------------+ 

但是，如果我從外部select中刪除RowNumber1列，結果似乎會改變。現在PaymentId的每個值都是1，即使我沒有觸及內部select語句。

SELECT CustomerId, PaymentId, RowNumber2 
FROM (
    SELECT 
     c.Id as CustomerId, 
     p.Id as PaymentId, 
     ROW_NUMBER() OVER (PARTITION BY p.Id ORDER BY p.Id) AS RowNumber1, 
     ROW_NUMBER() OVER (PARTITION BY c.Id ORDER BY c.Id) AS RowNumber2 
    FROM Customer as c 
    CROSS JOIN Payment as p 
) AS INNERSELECT WHERE RowNumber2 = 1 

+------------+-----------+------------+ 
| CustomerId | PaymentId | RowNumber2 | 
+------------+-----------+------------+ 
|   4 |   1 |   1 | 
|   5 |   1 |   1 | 
+------------+-----------+------------+ 

任何人都可以向我解釋爲什麼從外部select中刪除列會導致PaymentId列中的值發生更改？我可以使用其他什麼方法來實現我想要的目標，而不需要結果集中的行號？

Answer 1

這是因爲您的子查詢中的row_number（）順序通常沒有定義。 當你犯了一個交叉連接，行可以按任何順序

這可能是：

CUSTOMERID PAYMENTID 
     4   1  
     4   2  
     5   2  
     5   1 

，也可能是

CUSTOMERID PAYMENTID 
     4   1  
     4   2  
     5   1  
     5   2 

當你通過CUSTOMERID第一記錄分區計算ROW_NUMBER ，你得到

CUSTOMERID PAYMENTID ROWNUMBER 
     4   1   1 
     4   2   2 
     5   2   1 
     5   1   2 

當你計算第二個row_number記錄分區的CUSTOMERID，你

CUSTOMERID PAYMENTID ROWNUMBER 
     4   1   1 
     4   2   2 
     5   1   1 
     5   2   2 

如果你wnat剛剛配對隨機客戶和支付，你可以做到這一點

with cte_cust as (
    select id, row_number() over (order by id) as row_num 
    from Customer 
), cte_pay as (
    select id, row_number() over (order by id) as row_num 
    from Payment 
) 
select 
    c.id as CustomerId, 
    p.id as PaymentId 
from cte_cust as c 
    inner join cte_pay as p on p.row_num = c.row_num 

請注意，如果你有超過支付更多的客戶，有些客戶會不會出現在結果中（反之亦然）。

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