Null的大小写表达问题

Question

I am training on a northwind database but got stuck in a case statement, i am placing the query below and would appreciate assistance.

The issue is i want the records in column Shippeddate to have some text in this case i placed 'ddd' when there is NULL on it, but NULL's keep appearing in the result set.

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

try using ISNULL() instead:

so rather than

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

use:

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

so your query will be like:

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

Use WHEN ShippedDate IS NULL instead of CASE ShippedDate WHEN NULL :

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