Null的大小写表达问题

Question

我正在罗斯文（Northwind）数据库上进行培训，但被困在案例陈述中，我将查询放在下面，希望能为您提供帮助。

问题是我希望Shippeddate列中的记录有一些文本，在这种情况下，当它上有NULL时我将其放置为'ddd'，但NULL始终出现在结果集中。

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

尝试使用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')

因此您的查询将类似于：

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 
.....