Null的案件表達問題

Question

我在Northwind數據庫上進行培訓，但被困在案例陳述中，我在下面放置查詢並希望得到援助。

問題是我希望Shippeddate列中的記錄在這種情況下有一些文本，當它有NULL的時候我放置了'ddd'，但是NULL不斷出現在結果集中。

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

使用WHEN ShippedDate IS NULL，而不是CASE ShippedDate WHEN NULL：

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

Answer 2

嘗試使用ISNULL()代替：

因此而不是

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

使用：

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

所以您的查詢就會像：

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