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MYSQL數據庫MySQL 的CASE WHEN 語句使用說明

作者：VEPHP   時間 2017-09-28

《MYSQL數據庫MySQL 的CASE WHEN 語句使用說明》要點：
本文介紹了MYSQL數據庫MySQL 的CASE WHEN 語句使用說明，希望對您有用。如果有疑問，可以聯系我們。

MYSQL數據庫mysql數據庫中CASE WHEN語句.

MYSQL數據庫case when語句,用于計算條件列表并返回多個可能結果表達式之一.

MYSQL數據庫CASE 具有兩種格式：

MYSQL數據庫簡單 CASE 函數將某個表達式與一組簡單表達式進行比較以確定結果.

MYSQL數據庫CASE 搜索函數計算一組布爾表達式以確定結果.
兩種格式都支持可選的 ELSE 參數.

MYSQL數據庫語法
簡單 CASE 函數：

代碼如下:

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

MYSQL數據庫CASE 搜索函數：

代碼如下:

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

MYSQL數據庫參數
input_expression

MYSQL數據庫是使用簡單 CASE 格式時所計算的表達式.Input_expression 是任何有效的 Microsoft? SQL Server? 表達式.

MYSQL數據庫WHEN when_expression

MYSQL數據庫使用簡單 CASE 格式時 input_expression 所比較的簡單表達式.When_expression 是任意有效的 SQL Server 表達式.Input_expression 和每個 when_expression 的數據類型必須相同,或者是隱性轉換.

MYSQL數據庫占位符,表明可以使用多個 WHEN when_expression THEN result_expression 子句或 WHEN Boolean_expression THEN result_expression 子句.

MYSQL數據庫THEN result_expression

MYSQL數據庫當 input_expression = when_expression 取值為 TRUE,或者 Boolean_expression 取值為 TRUE 時返回的表達式.
result expression 是任意有效的 SQL Server 表達式.

MYSQL數據庫ELSE else_result_expression

MYSQL數據庫當比較運算取值不為 TRUE 時返回的表達式.如果省略此參數并且比較運算取值不為 TRUE,CASE 將返回 NULL 值.Else_result_expression 是任意有效的 SQL Server 表達式.Else_result_expression 和所有 result_expression 的數據類型必須相同,或者必須是隱性轉換.

MYSQL數據庫WHEN Boolean_expression

MYSQL數據庫使用 CASE 搜索格式時所計算的布爾表達式.Boolean_expression 是任意有效的布爾表達式.

MYSQL數據庫結果類型

MYSQL數據庫從 result_expressions 和可選 else_result_expression 的類型集合中返回最高的優先規則類型.有關更多信息,請參見數據類型的優先順序.

MYSQL數據庫結果值

MYSQL數據庫簡單 CASE 函數：
計算 input_expression,然后按指定順序對每個 WHEN 子句的 input_expression = when_expression 進行計算.

MYSQL數據庫返回第一個取值為 TRUE 的 (input_expression = when_expression) 的 result_expression.

MYSQL數據庫如果沒有取值為 TRUE 的 input_expression = when_expression,則當指定 ELSE 子句時 SQL Server 將返回 else_result_expression；若沒有指定 ELSE 子句,則返回 NULL 值.
CASE 搜索函數：
按指定順序為每個 WHEN 子句的 Boolean_expression 求值.

MYSQL數據庫返回第一個取值為 TRUE 的 Boolean_expression 的 result_expression.

MYSQL數據庫如果沒有取值為 TRUE 的 Boolean_expression,則當指定 ELSE 子句時 SQL Server 將返回 else_result_expression；若沒有指定 ELSE 子句,則返回 NULL 值.

MYSQL數據庫下面分享一些mysql case when語句的例子.

MYSQL數據庫A. 使用帶有簡單 CASE 函數的 SELECT 語句
在 SELECT 語句中,簡單 CASE 函數僅檢查是否相等,而不進行其它比較.

MYSQL數據庫例子,使用 CASE 函數更改圖書分類顯示.

代碼如下:

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

MYSQL數據庫注釋,后來我試了一下不讓用category=.

MYSQL數據庫我使用的代碼為：
?

代碼如下:

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

MYSQL數據庫結果集：
?

MYSQL數據庫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???????????????????

MYSQL數據庫???????????????????????????????????????????????? avg
???????????????????????????????????????????????? ==========================
???????????????????????????????????????????????? 13.73???????????????????

MYSQL數據庫Category?????????????? Shortened Title????????????? Price????????????????????????
------------------- ------------------------- --------------------------
Modern Cooking???????? The Gourmet Microwave??????? 2.99?????????????????????????
Modern Cooking???????? Silicon Valley Gastronomi 19.99???????????????????

MYSQL數據庫???????????????????????????????????????????????? avg
???????????????????????????????????????????????? ==========================
???????????????????????????????????????????????? 11.49???????????????????

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

MYSQL數據庫???????????????????????????????????????????????? avg
???????????????????????????????????????????????? ==========================
???????????????????????????????????????????????? 21.48???????????????????

MYSQL數據庫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???????????????????

MYSQL數據庫???????????????????????????????????????????????? avg
???????????????????????????????????????????????? ==========================
???????????????????????????????????????????????? 13.50???????????????????

MYSQL數據庫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???????????????????

MYSQL數據庫???????????????????????????????????????????????? avg
???????????????????????????????????????????????? ==========================
???????????????????????????????????????????????? 15.96???????????????????

MYSQL數據庫(21 row(s) affected)

MYSQL數據庫B. 使用帶有簡單 CASE 函數和 CASE 搜索函數的

MYSQL數據庫?SELECT 語句
在 SELECT 語句中,CASE 搜索函數允許根據比較值在結果集內對值進行替換.

MYSQL數據庫例子：根據圖書的價格范圍將價格(money 列)顯示為文本注釋.
?

代碼如下:

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

MYSQL數據庫結果集：
?

MYSQL數據庫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

MYSQL數據庫(18 row(s) affected)

MYSQL數據庫C. 使用帶有 SUBSTRING 和 SELECT 的 CASE 函數

MYSQL數據庫例子,使用 CASE 和 THEN 生成一個有關作者、圖書標識號和每個作者所著圖書類型的列表.
?

代碼如下:

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

MYSQL數據庫結果集：
?

MYSQL數據庫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 中被誤用最多的關鍵字之一.
雖然,可能以前用過這個關鍵字來創建字段,但是它還具有更多用法.

MYSQL數據庫例如,可以在 WHERE 子句中使用 CASE.

MYSQL數據庫首先,來看下 CASE 的語法.在一般的 SELECT 中,其語法如下：

代碼如下:

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

MYSQL數據庫以上代碼,需要用具體的參數代替尖括號中的內容.

MYSQL數據庫例子：

代碼如下:

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

MYSQL數據庫這是 CASE 的典型用法,但是使用 CASE 其實可以做更多的事情.

MYSQL數據庫比方說下面的 GROUP BY 子句中的 CASE：

代碼如下:

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

甚至還可以組合這些選項,添加一個 ORDER BY 子句,例如：
?
代碼如下:

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

MYSQL數據庫注意,為了在 GROUP BY 塊中使用 CASE,查詢語句需要在 GROUP BY 塊中重復 SELECT 塊中的 CASE 塊.

MYSQL數據庫除了選擇自定義字段之外,在很多情況下 CASE 都非常有用.

MYSQL數據庫稍加深入,還可以得到以前認為不可能得到的分組排序結果集.

使用CASE WHEN進行字符串替換處理
代碼如下:

/*
?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;

簡單語句
代碼如下:

SELECT CASE WHEN 10*2=30 THEN '30 correct'
WHEN 10*2=40 THEN '40 correct'
ELSE 'Should be 10*2=20'
END;

多重表達式
代碼如下:

SELECT CASE 10*2
WHEN 20 THEN '20 correct'
WHEN 30 THEN '30 correct'
WHEN 40 THEN '40 correct'
END;

在SELECT查詢中使用CASE WHEN
代碼如下:

/*
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;

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