Null的大小寫表達問題

Question

我正在羅斯文（Northwind）數據庫上進行培訓，但被困在案例陳述中，我將查詢放在下面，希望能為您提供幫助。

問題是我希望Shippeddate列中的記錄有一些文本，在這種情況下，當它上有NULL時我將其放置為'ddd'，但NULL始終出現在結果集中。

SELECT OrderID, 
       CONVERT(VARCHAR(10), OrderDate, 103)    AS OrderDate, 
       CONVERT(VARCHAR(10), RequiredDate, 103) AS RequiredDate, 
       CASE ShippedDate 
         WHEN NULL THEN 'ddd' 
         ELSE CONVERT(VARCHAR(10), ShippedDate, 103) 
       END                                     AS ShippedDate 
       -- Why is this not working 
       , 
       [UnitPrice]                             AS UnitPriceOnOrder, 
       [Quantity]                              AS QuantityOnOrder, 
       [Discount]                              AS DiscountOnOrder, 
       CompanyName_II                          AS CustomerCompanyName, 
       ContactName_II                          AS CustomerContact, 
       ContactTitle_II                         AS CustomerContactTitle, 
       City_II                                 AS CustomerCity, 
       Country_II                              AS CustomerCountry, 
       ProductName, 
       CategoryName, 
       [Description]                           AS CategoryDescription, 
       UnitsInStock, 
       UnitsOnOrder, 
       UnitsInStock - UnitsOnOrder             AS AvailableUnitsInStock, 
       FirstName + ' ' + LastName              AS EmployeeName 
FROM   ##NorthwindTestI 
WHERE  [Quantity] > 10 
       AND ShippedDate > '19970101' 
        OR [UnitPrice] > 10 
        OR Country_II = 'USA' 
        OR FirstName + ' ' + LastName = 'Janet Leverling' 
ORDER  BY 
--[Quantity] 
ShippedDate 

Answer 1

嘗試使用ISNULL()代替：

所以而不是

CASE ShippedDate 
         WHEN NULL THEN 'ddd' 
         ELSE CONVERT(VARCHAR(10), ShippedDate, 103) 
       END                                     AS ShippedDate 

采用：

ISNULL(CONVERT(VARCHAR(10),ShippedDate,103),'ddd')

因此您的查詢將類似於：

SELECT OrderID, 
       CONVERT(VARCHAR(10), OrderDate, 103)    AS OrderDate, 
       CONVERT(VARCHAR(10), RequiredDate, 103) AS RequiredDate, 
       ISNULL(CONVERT(VARCHAR(10),ShippedDate,103),'ddd') AS ShippedDate 
       -- Why is this not working 
       , 
       [UnitPrice]                             AS UnitPriceOnOrder, 
       [Quantity]                              AS QuantityOnOrder, 
       [Discount]                              AS DiscountOnOrder, 
       CompanyName_II                          AS CustomerCompanyName, 
       ContactName_II                          AS CustomerContact, 
       ContactTitle_II                         AS CustomerContactTitle, 
       City_II                                 AS CustomerCity, 
       Country_II                              AS CustomerCountry, 
       ProductName, 
       CategoryName, 
       [Description]                           AS CategoryDescription, 
       UnitsInStock, 
       UnitsOnOrder, 
       UnitsInStock - UnitsOnOrder             AS AvailableUnitsInStock, 
       FirstName + ' ' + LastName              AS EmployeeName 
FROM   ##NorthwindTestI 
WHERE  [Quantity] > 10 
       AND ShippedDate > '19970101' 
        OR [UnitPrice] > 10 
        OR Country_II = 'USA' 
        OR FirstName + ' ' + LastName = 'Janet Leverling' 
ORDER  BY 
--[Quantity] 
ShippedDate 

Answer 2

使用WHEN ShippedDate IS NULL代替CASE ShippedDate WHEN NULL ：

SELECT .....,
       CASE WHEN ShippedDate IS NULL  
        THEN 'ddd' 
        ELSE CONVERT(VARCHAR(10), ShippedDate, 103) 
       END  AS ShippedDate 
.....