Question

我正在北风数据库上进行培训但遇到了一个案例陈述，我在下面提出查询并希望得到帮助。

问题是我希望Shippeddate列中的记录包含一些文本，在这种情况下，我放置了＆＃39; ddd＆＃39;当它上面有NULL时，NULL会一直出现在结果集中。

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

使用：

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

因此您的查询将如下：

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 
.....

Null的案例表达问题

2 个答案: