sql:字段累加（int ,varchar）避免Null+"文本"=Null

sql:字段累加（int ,varchar）避免Null+"文本"=Null
比如 OutedTIDs varchar (50)  NuLL

当OutedTIDs=Null时：
sql： Update 表 set OutedTIDs= OutedTIDs + '合肥,'    
结果：Null


sql：Update 表 set OutedTIDs=case when OutedTIDs is null then '' else OutedTIDs end+ '合肥,'  
结果：合肥,

使用 CASE 处理条件数据

CASE 函数用于计算多个条件并为每个条件返回单个值。CASE 函数通常的用途是使用可读性更强的值替换代码或缩写。下面的查询使用 CASE 函数重命名书籍的分类，以使之更易理解。

USE pubs
SELECT
   CASE type
      WHEN 'popular_comp' THEN 'Popular Computing'
      WHEN 'mod_cook' THEN 'Modern Cooking'
      WHEN 'business' THEN 'Business'
      WHEN 'psychology' THEN 'Psychology'
      WHEN 'trad_cook' THEN 'Traditional Cooking'
      ELSE 'Not yet categorized'
   END AS Category, 
CONVERT(varchar(30), title) AS "Shortened Title", 
price AS Price
FROM titles
WHERE price IS NOT NULL
ORDER BY 1

下面是结果集：

category            shortened title                Price   
------------------- ------------------------------ ------- 
Business            Cooking with Computers: Surrep 11.95   
Business            Straight Talk About Computers  19.99   
Business            The Busy Executive's Database  19.99   
Business            You Can Combat Computer Stress 2.99    
Modern Cooking      Silicon Valley Gastronomic Tre 19.99   
Modern Cooking      The Gourmet Microwave          2.99    
Popular Computing   But Is It User Friendly?       22.95   
Popular Computing   Secrets of Silicon Valley      20.00   
Psychology          Computer Phobic AND Non-Phobic 21.59   
Psychology          Emotional Security: A New Algo 7.99    
Psychology          Is Anger the Enemy?            10.95   
Psychology          Life Without Fear              7.00    
Psychology          Prolonged Data Deprivation: Fo 19.99   
Traditional Cooking Fifty Years in Buckingham Pala 11.95   
Traditional Cooking Onions, Leeks, and Garlic: Coo 20.95   
Traditional Cooking Sushi, Anyone?                 14.99   

(16 row(s) affected)

CASE 函数的另一个用途给数据分类。下面的查询使用 CASE 函数对价格分类。

SELECT
   CASE 
      WHEN price IS NULL THEN 'Not yet priced'
      WHEN price < 10 THEN 'Very Reasonable Title'
      WHEN price >= 10 and price < 20 THEN 'Coffee Table Title'
      ELSE 'Expensive book!'
   END AS "Price Category",
CONVERT(varchar(20), title) AS "Shortened Title"
FROM pubs.dbo.titles
ORDER BY price

下面是结果集：

Price Category        Shortened Title      
--------------------- -------------------- 
Not yet priced        The Psychology of Co 
Not yet priced        Net Etiquette        
Very Reasonable Title You Can Combat Compu 
Very Reasonable Title The Gourmet Microwav 
Very Reasonable Title Life Without Fear    
Very Reasonable Title Emotional Security:  
Coffee Table Title    Is Anger the Enemy?  
Coffee Table Title    Cooking with Compute 
Coffee Table Title    Fifty Years in Bucki 
Coffee Table Title    Sushi, Anyone?       
Coffee Table Title    The Busy Executive's 
Coffee Table Title    Straight Talk About  
Coffee Table Title    Silicon Valley Gastr 
Coffee Table Title    Prolonged Data Depri 
Expensive book!       Secrets of Silicon V 
Expensive book!       Onions, Leeks, and G 
Expensive book!       Computer Phobic AND  
Expensive book!       But Is It User Frien 

(18 row(s) affected)