MySQL 的CASE WHEN 语句使用说明

2022-11-12 09:23:22

内容摘要

这篇文章主要为大家详细介绍了MySQL 的CASE WHEN 语句使用说明，具有一定的参考价值，可以用来参考一下。对此感兴趣的朋友，看看idc笔记做的技术笔记！ mysql数据库中CASE WHEN语

文章正文

这篇文章主要为大家详细介绍了MySQL 的CASE WHEN 语句使用说明，具有一定的参考价值，可以用来参考一下。

对此感兴趣的朋友，看看idc笔记做的技术笔记！

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：

代码如下:


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

CASE 搜索函数：

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。

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：

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 2.99Business Cooking with Computers: S 11.95Business The Busy Executive's Data 19.99Business Straight Talk About Compu 19.99

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

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

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

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

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

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

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

Category Shortened Title Price------------------- ------------------------- --------------------------Traditional Cooking Fifty Years in Buckingham 11.95Traditional Cooking Sushi, Anyone? 14.99Traditional 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。

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 EtiquetteNot yet priced The Psychology of CoVery Reasonable Title The Gourmet MicrowavVery Reasonable Title You Can Combat CompuVery Reasonable Title Life Without FearVery Reasonable Title Emotional Security:Coffee Table Title Is Anger the Enemy?Coffee Table Title Cooking with ComputeCoffee Table Title Fifty Years in BuckiCoffee Table Title Sushi, Anyone?Coffee Table Title Prolonged Data DepriCoffee Table Title Silicon Valley GastrCoffee Table Title Straight Talk AboutCoffee Table Title The Busy Executive'sExpensive book! Secrets of Silicon VExpensive book! Onions, Leeks, and GExpensive book! Computer Phobic AndExpensive 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。

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 PsychologyMarjorie Green 213-46-8915 BU1032 BusinessMarjorie Green 213-46-8915 BU2075 BusinessCheryl Carson 238-95-7766 PC1035 Popular ComputingMichael O'Leary 267-41-2394 BU1111 BusinessMichael O'Leary 267-41-2394 TC7777 Traditional CookingDean Straight 274-80-9391 BU7832 BusinessAbraham Bennet 409-56-7008 BU1032 BusinessAnn Dull 427-17-2319 PC8888 Popular ComputingBurt Gringlesby 472-27-2349 TC7777 Traditional CookingCharlene Locksley 486-29-1786 PC9999 Popular ComputingCharlene Locksley 486-29-1786 PS7777 PsychologyReginald Blotchet-Halls 648-92-1872 TC4203 Traditional CookingAkiko Yokomoto 672-71-3249 TC7777 Traditional CookingInnes del Castillo 712-45-1867 MC2222 Modern CookingMichel DeFrance 722-51-5454 MC3021 Modern CookingStearns MacFeather 724-80-9391 BU1111 BusinessStearns MacFeather 724-80-9391 PS1372 PsychologyLivia Karsen 756-30-7391 PS1372 PsychologySylvia Panteley 807-91-6654 TC3218 Traditional CookingSheryl Hunter 846-92-7186 PC8888 Popular ComputingAnne Ringer 899-46-2035 MC3021 Modern CookingAnne Ringer 899-46-2035 PS2091 PsychologyAlbert Ringer 998-72-3567 PS2091 PsychologyAlbert 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：

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：

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：

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：

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

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

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

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

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;

注：关于MySQL 的CASE WHEN 语句使用说明的内容就先介绍到这里，更多相关文章的可以留意

代码注释

上一篇 MySQL 队列实现并发读下一篇 MySQL中将null值转换为0的语句

作者：喵哥笔记

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