如何在SQL查詢中使用計算列

Question

我不知道這個問題的標題是否明白，但是我的意思是這個。

我有一張銷售表，我需要對表進行“格式化”，並在查詢中添加一些列以指示總數，變更率（我從另一張表中取出）和以其他貨幣計的總金額。

所以我的查詢看起來像：

SELECT SalesID,SalesType,SalesDate,CurrencyType,
       case when CurrencyType = 1 then Total else 0 AS TotalOnLocalCurrency,
       case when CurrencyType = 2 then Total else 0 AS TotalOnOtherCurrency,
      (TotalOnLocalCurrency/dbo.GetCurrencyRate(SalesDate)) + TotalOnOtherCurrency) Total
FROM Sales

如果可能出現這種情況，我是說我嘗試執行此操作，但出現此錯誤：

無效的列名“ TotalOnLocalCurrency”。

Answer 1

您可以將整個公式放在后面的行中，或為此使用CROSS APPPLY ：

SELECT SalesID,
       SalesType,
       SalesDate,
       CurrencyType,
       TotalOnLocalCurrency,
       TotalOnOtherCurrency,
      (TotalOnLocalCurrency/dbo.GetCurrencyRate(SalesDate)) + TotalOnOtherCurrency Total
FROM Sales
CROSS APPLY (SELECT CASE WHEN CurrencyType = 1 then Total else 0) CxA(TotalOnLocalCurrency)
CROSS APPLY (SELECT case when CurrencyType = 2 then Total else 0) CxB(TotalOnOtherCurrency)

Answer 2

您收到此錯誤是因為您已對表達式CurrencyType = 1 then Total else 0 AS TotalOnLocalCurrency別名， CurrencyType = 1 then Total else 0 AS TotalOnLocalCurrency對別名別名CurrencyType = 1 then Total else 0 AS TotalOnLocalCurrency 。 您不能在同一select子句中引用別名列名稱，這違反了邏輯查詢處理規則。 相反，可以引用表達式本身，也可以將表達式包裝在表表達式中，然后查詢表表達式。

Answer 3

您不能使用在同一查詢中引入的列別名。 如果您使用的是SQL Server 2005及更高版本，則可以使用CTE（公用表表達式）來處理。

;WITH SalesData AS
(
    SELECT 
        SalesID, SalesType, SalesDate, CurrencyType,
        TotalOnLocalCurrency = CASE WHEN CurrencyType = 1 THEN Total ELSE 0 END,
        TotalOnOtherCurrency = CASE WHEN CurrencyType = 2 THEN Total ELSE END 
    FROM Sales
)
SELECT 
   SalesID, SalesType, SalesDate, CurrencyType,
   TotalOnLocalCurrency, TotalOnOtherCurrency,
   Total = (TotalOnLocalCurrency/dbo.GetCurrencyRate(SalesDate)) + TotalOnOtherCurrency) 
FROM  
   SalesData

Answer 4

嘗試更多類似的方法：

select 
SalesID,SalesType,SalesDate,CurrencyType,
TotalOnLocalCurrency,TotalOnOtherCurrency,
TotalOnLocalCurrency/dbo.GetCurrencyRate(SalesDate)) + TotalOnOtherCurrency as total from
(
    SELECT SalesID,SalesType,SalesDate,CurrencyType,
           case when CurrencyType = 1 then Total else 0 END AS TotalOnLocalCurrency,
           case when CurrencyType = 2 then Total else 0 END AS TotalOnOtherCurrency
    FROM Sales) t