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mysql中case when用法与及注意事项_mysql when case

mysql when case

CASE
计算条件列表并返回多个可能结果表达式之一。

CASE 具有两种格式:

简单 CASE 函数将某个表达式与一组简单表达式进行比较以确定结果。


CASE 搜索函数计算一组布尔表达式以确定结果。 
两种格式都支持可选的 ELSE 参数。

语法
简单 CASE 函数:

CASE input_expression
    WHEN when_expression THEN result_expression
        [ ...n ]
    [ 
        ELSE else_result_expression
    END

CASE 搜索函数:

CASE
    WHEN Boolean_expression THEN result_expression
        [ ...n ]
    [ 
        ELSE else_result_expression
    END

参数
input_expression

是使用简单 CASE 格式时所计算的表达式。Input_expression 是任何有效的 Microsoft® SQL Server™ 表达式。

WHEN when_expression

使用简单 CASE 格式时 input_expression 所比较的简单表达式。When_expression 是任意有效的 SQL Server 表达式。Input_expression 和每个 when_expression 的数据类型必须相同,或者是隐性转换。

n

占位符,表明可以使用多个 WHEN when_expression THEN result_expression 子句或 WHEN Boolean_expression THEN result_expression 子句。

THEN result_expression

当 input_expression = when_expression 取值为 TRUE,或者 Boolean_expression 取值为 TRUE 时返回的表达式。result expression 是任意有效的 SQL Server 表达式。

ELSE else_result_expression

当比较运算取值不为 TRUE 时返回的表达式。如果省略此参数并且比较运算取值不为 TRUE,CASE 将返回 NULL 值。Else_result_expression 是任意有效的 SQL Server 表达式。Else_result_expression 和所有 result_expression 的数据类型必须相同,或者必须是隐性转换。

WHEN Boolean_expression

使用 CASE 搜索格式时所计算的布尔表达式。Boolean_expression 是任意有效的布尔表达式。

结果类型
从 result_expressions 和可选 else_result_expression 的类型集合中返回最高的优先规则类型。有关更多信息,请参见数据类型的优先顺序。

结果值
简单 CASE 函数:
计算 input_expression,然后按指定顺序对每个 WHEN 子句的 input_expression = when_expression 进行计算。


返回第一个取值为 TRUE 的 (input_expression = when_expression) 的 result_expression。


如果没有取值为 TRUE 的 input_expression = when_expression,则当指定 ELSE 子句时 SQL Server 将返回 else_result_expression;若没有指定 ELSE 子句,则返回 NULL 值。 
CASE 搜索函数:
按指定顺序为每个 WHEN 子句的 Boolean_expression 求值。


返回第一个取值为 TRUE 的 Boolean_expression 的 result_expression。


如果没有取值为 TRUE 的 Boolean_expression,则当指定 ELSE 子句时 SQL Server 将返回 else_result_expression;若没有指定 ELSE 子句,则返回 NULL 值。 
示例
A. 使用带有简单 CASE 函数的 SELECT 语句
在 SELECT 语句中,简单 CASE 函数仅检查是否相等,而不进行其它比较。下面的示例使用 CASE 函数更改图书分类显示,以使其更易于理解。

USE pubs
GO
SELECT      Category = 
         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,
      CAST(title AS varchar(25)) AS 'Shortened Title',
      price AS Price
FROM titles
WHERE price IS NOT NULL
ORDER BY type, price
COMPUTE AVG(price) BY type
GO

注释,后来我试了一下不让用category=

我的代码是

SELECT  
  case gender
 WHEN 1 THEN 'NAN'
 WHEN 0 THEN 'NV'
end  as gender
FROM
t_swidy_day_nutrient

下面是结果集:

 

Category            Shortened Title           Price                      ------------------- ------------------------- -------------------------- Business            You Can Combat Computer S 2.99                       Business            Cooking with Computers: S 11.95                      Business            The Busy Executive's Data 19.99                      Business            Straight Talk About Compu 19.99                     

                                              avg                                               ==========================                                               13.73                     

Category            Shortened Title           Price                      ------------------- ------------------------- -------------------------- Modern Cooking      The Gourmet Microwave     2.99                       Modern Cooking      Silicon Valley Gastronomi 19.99                     

                                              avg                                               ==========================                                               11.49                     

Category            Shortened Title           Price                      ------------------- ------------------------- -------------------------- Popular Computing   Secrets of Silicon Valley 20.00                      Popular Computing   But Is It User Friendly? 22.95                     

                                              avg                                               ==========================                                               21.48                     

Category            Shortened Title           Price                      ------------------- ------------------------- -------------------------- Psychology          Life Without Fear         7.00                       Psychology          Emotional Security: A New 7.99                       Psychology          Is Anger the Enemy?       10.95                      Psychology          Prolonged Data Deprivatio 19.99                      Psychology          Computer Phobic AND Non-P 21.59                     

                                              avg                                               ==========================                                               13.50                     

Category            Shortened Title           Price                      ------------------- ------------------------- -------------------------- Traditional Cooking Fifty Years in Buckingham 11.95                      Traditional Cooking Sushi, Anyone?            14.99                      Traditional Cooking Onions, Leeks, and Garlic 20.95                     

                                              avg                                               ==========================                                               15.96                     

(21 row(s) affected)

B. 使用带有简单 CASE 函数和 CASE 搜索函数的 SELECT 语句
在 SELECT 语句中,CASE 搜索函数允许根据比较值在结果集内对值进行替换。下面的示例根据图书的价格范围将价格(money 列)显示为文本注释。

USE pubs
GO
SELECT       'Price Category' = 
         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,
      CAST(title AS varchar(20)) AS 'Shortened Title'
FROM titles
ORDER BY price
GO

下面是结果集:

 

Price Category        Shortened Title      --------------------- -------------------- Not yet priced        Net Etiquette        Not yet priced        The Psychology of Co Very Reasonable Title The Gourmet Microwav Very Reasonable Title You Can Combat Compu 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    Prolonged Data Depri Coffee Table Title    Silicon Valley Gastr Coffee Table Title    Straight Talk About Coffee Table Title    The Busy Executive's 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)

C. 使用带有 SUBSTRING 和 SELECT 的 CASE 函数
下面的示例使用 CASE 和 THEN 生成一个有关作者、图书标识号和每个作者所著图书类型的列表。

USE pubs
SELECT SUBSTRING((RTRIM(a.au_fname) + ' '+ 
      RTRIM(a.au_lname) + ' '), 1, 25) AS Name, a.au_id, ta.title_id,
      Type = 
  CASE 
       WHEN SUBSTRING(ta.title_id, 1, 2) = 'BU' THEN 'Business'
       WHEN SUBSTRING(ta.title_id, 1, 2) = 'MC' THEN 'Modern Cooking'
       WHEN SUBSTRING(ta.title_id, 1, 2) = 'PC' THEN 'Popular Computing'
       WHEN SUBSTRING(ta.title_id, 1, 2) = 'PS' THEN 'Psychology'
       WHEN SUBSTRING(ta.title_id, 1, 2) = 'TC' THEN 'Traditional Cooking'
  END
FROM titleauthor ta JOIN authors a ON ta.au_id = a.au_id

下面是结果集:

   

Name                         au_id          title_id Type                   
------------------------- ----------- -------- ------------------- 
Johnson White                172-32-1176 PS3333      Psychology             
Marjorie Green               213-46-8915 BU1032      Business               
Marjorie Green               213-46-8915 BU2075      Business               
Cheryl Carson                238-95-7766 PC1035      Popular Computing      
Michael O'Leary              267-41-2394 BU1111      Business               
Michael O'Leary              267-41-2394 TC7777      Traditional Cooking 
Dean Straight                274-80-9391 BU7832      Business               
Abraham Bennet               409-56-7008 BU1032      Business               
Ann Dull                     427-17-2319 PC8888      Popular Computing      
Burt Gringlesby              472-27-2349 TC7777      Traditional Cooking 
Charlene Locksley            486-29-1786 PC9999      Popular Computing      
Charlene Locksley            486-29-1786 PS7777      Psychology             
Reginald Blotchet-Halls      648-92-1872 TC4203      Traditional Cooking 
Akiko Yokomoto               672-71-3249 TC7777      Traditional Cooking 
Innes del Castillo           712-45-1867 MC2222      Modern Cooking         
Michel DeFrance              722-51-5454 MC3021      Modern Cooking         
Stearns MacFeather           724-80-9391 BU1111      Business               
Stearns MacFeather           724-80-9391 PS1372      Psychology             
Livia Karsen                 756-30-7391 PS1372      Psychology             
Sylvia Panteley              807-91-6654 TC3218      Traditional Cooking 
Sheryl Hunter                846-92-7186 PC8888      Popular Computing      
Anne Ringer                  899-46-2035 MC3021      Modern Cooking         
Anne Ringer                  899-46-2035 PS2091      Psychology             
Albert Ringer                998-72-3567 PS2091      Psychology             
Albert Ringer                998-72-3567 PS2106      Psychology  
(25 row(s) affected)

   

CASE 可能是 SQL 中被误用最多的关键字之一。虽然你可能以前用过这个关键字来创建字段,但是它还具有更多用法。例如,你可以在 WHERE 子句中使用 CASE。

首先让我们看一下 CASE 的语法。在一般的 SELECT 中,其语法如下:

SELECT <myColumnSpec> = 
CASE 
WHEN <A> THEN <somethingA> 
WHEN <B> THEN <somethingB> 
ELSE <somethingE> 
END

在上面的代码中需要用具体的参数代替尖括号中的内容。下面是一个简单的例子:

USE pubs 
GO 
SELECT 
    Title, 
    'Price Range' = 
    CASE 
        WHEN price IS NULL THEN 'Unpriced' 
        WHEN price < 10 THEN 'Bargain' 
        WHEN price BETWEEN 10 and 20 THEN 'Average' 
        ELSE 'Gift to impress relatives' 
    END 
FROM titles 
ORDER BY price 
GO

这是 CASE 的典型用法,但是使用 CASE 其实可以做更多的事情。比方说下面的 GROUP BY 子句中的 CASE:

SELECT 'Number of Titles', Count(*) 
FROM titles 
GROUP BY 
    CASE 
        WHEN price IS NULL THEN 'Unpriced' 
        WHEN price < 10 THEN 'Bargain' 
        WHEN price BETWEEN 10 and 20 THEN 'Average' 
        ELSE 'Gift to impress relatives' 
    END 
GO

你甚至还可以组合这些选项,添加一个 ORDER BY 子句,如下所示:

USE pubs 
GO 
SELECT 
    CASE 
        WHEN price IS NULL THEN 'Unpriced' 
        WHEN price < 10 THEN 'Bargain' 
        WHEN price BETWEEN 10 and 20 THEN 'Average' 
        ELSE 'Gift to impress relatives' 
    END AS Range, 
    Title 
FROM titles 
GROUP BY 
    CASE 
        WHEN price IS NULL THEN 'Unpriced' 
        WHEN price < 10 THEN 'Bargain' 
        WHEN price BETWEEN 10 and 20 THEN 'Average' 
        ELSE 'Gift to impress relatives' 
    END, 
    Title 
ORDER BY 
    CASE 
        WHEN price IS NULL THEN 'Unpriced' 
        WHEN price < 10 THEN 'Bargain' 
        WHEN price BETWEEN 10 and 20 THEN 'Average' 
        ELSE 'Gift to impress relatives' 
    END, 
    Title 
GO

注意,为了在 GROUP BY 块中使用 CASE,查询语句需要在 GROUP BY 块中重复 SELECT 块中的 CASE 块。

除了选择自定义字段之外,在很多情况下 CASE 都非常有用。再深入一步,你还可以得到你以前认为不可能得到的分组排序结果集。

需要注意的是,假如表中没有数据记录,并且在查询中又指明表名,则无论case  when  后有没有else  ,都返回null​​​​​​​

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