mysql数据库中CASE WHEN语句。

case when语句，用于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：

复制代码 代码如下:
CASE input_expression
    WHEN when_expression THEN result_expression
        [ ...n ]
    [
        ELSE else_result_expression
    END

CASE 搜索函数：

复制代码 代码如下:
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。

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。

下面分享一些mysql case when语句的例子。

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。

例子，使用 CASE 函数更改图书分类显示。

复制代码 代码如下:
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=。

我使用的代码为：
 

复制代码 代码如下:
SELECT 
  case gender
 WHEN 1 THEN 'NAN'
 WHEN 0 THEN 'NV'
end  as gender
FROM
t_swidy_day_nutrient

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。

例子：根据图书的价格范围将价格（money 列）显示为文本注释。
 

复制代码 代码如下:
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

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

例子，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。
 

复制代码 代码如下:
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

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，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，可以在 WHERE 子句中使用 CASE。

首先，来看下 CASE 的语法。在一般的 SELECT 中，其语法如下：

复制代码 代码如下:
SELECT <myColumnSpec> =
CASE
WHEN <A> THEN <somethingA>
WHEN <B> THEN <somethingB>
ELSE <somethingE>
END

以上代码，需要用具体的参数代替尖括号中的内容。

例子：

复制代码 代码如下:
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：

复制代码 代码如下:
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

甚至还可以组合这些选项，添加一个 ORDER BY 子句，例如：
 

复制代码 代码如下:
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。

稍加深入，还可以得到以前认为不可能得到的分组排序结果集。

使用CASE WHEN进行字符串替换处理
复制代码 代码如下:
/*
 mysql> select * from sales;
 +-----+------------+--------+--------+--------+------+------------+
 | num | name       | winter | spring | summer | fall | category   |
 +-----+------------+--------+--------+--------+------+------------+
 |   1 | Java       |   1067 |    200 |    150 |  267 | Holiday    |
 |   2 | C          |    970 |    770 |    531 |  486 | Profession |
 |   3 | JavaScript |     53 |     13 |     21 |  856 | Literary   |
 |   4 | SQL        |    782 |    357 |    168 |  250 | Profession |
 |   5 | Oracle     |    589 |    795 |    367 |  284 | Holiday    |
 |   6 | MySQL      |    953 |    582 |    336 |  489 | Literary   |
 |   7 | Cplus      |    752 |    657 |    259 |  478 | Literary   |
 |   8 | Python     |     67 |     23 |     83 |  543 | Holiday    |
 |   9 | PHP        |    673 |     48 |    625 |   52 | Profession |
 +-----+------------+--------+--------+--------+------+------------+
 9 rows in set (0.01 sec)
 mysql> SELECT name AS Name,
     -> CASE category
     -> WHEN "Holiday" THEN "Seasonal"
     -> WHEN "Profession" THEN "Bi_annual"
     -> WHEN "Literary" THEN "Random" END AS "Pattern"
     -> FROM sales;
 +------------+-----------+
 | Name       | Pattern   |
 +------------+-----------+
 | Java       | Seasonal  |
 | C          | Bi_annual |
 | JavaScript | Random    |
 | SQL        | Bi_annual |
 | Oracle     | Seasonal  |
 | MySQL      | Random    |
 | Cplus      | Random    |
 | Python     | Seasonal  |
 | PHP        | Bi_annual |
 +------------+-----------+
 9 rows in set (0.00 sec)
 */
 Drop table sales;
 CREATE TABLE sales(
     num MEDIUMINT NOT NULL AUTO_INCREMENT,
     name CHAR(20),
     winter INT,
     spring INT,
     summer INT,
     fall INT,
     category CHAR(13),
     primary key(num)
 )type=MyISAM;
 insert into sales value(1, 'Java', 1067 , 200, 150, 267,'Holiday');
 insert into sales value(2, 'C',970,770,531,486,'Profession');
 insert into sales value(3, 'JavaScript',53,13,21,856,'Literary');
 insert into sales value(4, 'SQL',782,357,168,250,'Profession');
 insert into sales value(5, 'Oracle',589,795,367,284,'Holiday');
 insert into sales value(6, 'MySQL',953,582,336,489,'Literary');
 insert into sales value(7, 'Cplus',752,657,259,478,'Literary');
 insert into sales value(8, 'Python',67,23,83,543,'Holiday');
 insert into sales value(9, 'PHP',673,48,625,52,'Profession');
 select * from sales;
 SELECT name AS Name,
 CASE category
 WHEN "Holiday" THEN "Seasonal"
 WHEN "Profession" THEN "Bi_annual"
 WHEN "Literary" THEN "Random" END AS "Pattern"
 FROM sales;

简单语句
复制代码 代码如下:
SELECT CASE WHEN 10*2=30 THEN '30 correct'
WHEN 10*2=40 THEN '40 correct'
ELSE 'Should be 10*2=20'
END;

多重表达式
复制代码 代码如下:
SELECT CASE 10*2
WHEN 20 THEN '20 correct'
WHEN 30 THEN '30 correct'
WHEN 40 THEN '40 correct'
END;

在SELECT查询中使用CASE WHEN
复制代码 代码如下:
/*
mysql> SELECT Name, RatingID AS Rating,
-> CASE RatingID
-> WHEN 'R' THEN 'Under 17 requires an adult.'
-> WHEN 'X' THEN 'No one 17 and under.'
-> WHEN 'NR' THEN 'Use discretion when renting.'
-> ELSE 'OK to rent to minors.'
-> END AS Policy
-> FROM DVDs
-> ORDER BY Name;
+-----------+--------+------------------------------+
| Name | Rating | Policy |
+-----------+--------+------------------------------+
| Africa | PG | OK to rent to minors. |
| Amadeus | PG | OK to rent to minors. |
| Christmas | NR | Use discretion when renting. |
| Doc | G | OK to rent to minors. |
| Falcon | NR | Use discretion when renting. |
| Mash | R | Under 17 requires an adult. |
| Show | NR | Use discretion when renting. |
| View | NR | Use discretion when renting. |
+-----------+--------+------------------------------+
8 rows in set (0.01 sec)
*/
Drop table DVDs;
CREATE TABLE DVDs (
ID SMALLINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
Name VARCHAR(60) NOT NULL,
NumDisks TINYINT NOT NULL DEFAULT 1,
RatingID VARCHAR(4) NOT NULL,
StatID CHAR(3) NOT NULL
)
ENGINE=INNODB;
INSERT INTO DVDs (Name, NumDisks, RatingID, StatID)
VALUES ('Christmas', 1, 'NR', 's1'),
('Doc', 1, 'G', 's2'),
('Africa', 1, 'PG', 's1'),
('Falcon', 1, 'NR', 's2'),
('Amadeus', 1, 'PG', 's2'),
('Show', 2, 'NR', 's2'),
('View', 1, 'NR', 's1'),
('Mash', 2, 'R', 's2');
SELECT Name, RatingID AS Rating,
CASE RatingID
WHEN 'R' THEN 'Under 17 requires an adult.'
WHEN 'X' THEN 'No one 17 and under.'
WHEN 'NR' THEN 'Use discretion when renting.'
ELSE 'OK to rent to minors.'
END AS Policy
FROM DVDs
ORDER BY Name;