MySQL 的CASE WHEN 語句使用說明
mysql數(shù)據(jù)庫中CASE WHEN語句。
case when語句，用于計算條件列表并返回多個可能結(jié)果表達(dá)式之一。
CASE 具有兩種格式：
簡單 CASE 函數(shù)將某個表達(dá)式與一組簡單表達(dá)式進(jìn)行比較以確定結(jié)果。
CASE 搜索函數(shù)計算一組布爾表達(dá)式以確定結(jié)果。
兩種格式都支持可選的 ELSE 參數(shù)。
語法
簡單 CASE 函數(shù)：
CASE input_expression
WHEN when_expression THEN result_expression
[ ...n ]
[
ELSE else_result_expression
END
CASE 搜索函數(shù)：
CASE
WHEN Boolean_expression THEN result_expression
[ ...n ]
[
ELSE else_result_expression
END
參數(shù)
input_expression
是使用簡單 CASE 格式時所計算的表達(dá)式。Input_expression 是任何有效的 Microsoft? SQL Server? 表達(dá)式。
WHEN when_expression
使用簡單 CASE 格式時 input_expression 所比較的簡單表達(dá)式。When_expression 是任意有效的 SQL Server 表達(dá)式。Input_expression 和每個 when_expression 的數(shù)據(jù)類型必須相同，或者是隱性轉(zhuǎn)換。
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。
THEN result_expression
當(dāng) input_expression = when_expression 取值為 TRUE，或者 Boolean_expression 取值為 TRUE 時返回的表達(dá)式。
result expression 是任意有效的 SQL Server 表達(dá)式。
ELSE else_result_expression
當(dāng)比較運算取值不為 TRUE 時返回的表達(dá)式。如果省略此參數(shù)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(dá)式。Else_result_expression 和所有 result_expression 的數(shù)據(jù)類型必須相同，或者必須是隱性轉(zhuǎn)換。
WHEN Boolean_expression
使用 CASE 搜索格式時所計算的布爾表達(dá)式。Boolean_expression 是任意有效的布爾表達(dá)式。
結(jié)果類型
從 result_expressions 和可選 else_result_expression 的類型集合中返回最高的優(yōu)先規(guī)則類型。有關(guān)更多信息，請參見數(shù)據(jù)類型的優(yōu)先順序。
結(jié)果值
簡單 CASE 函數(shù)：
計算 input_expression，然后按指定順序?qū)γ總€ WHEN 子句的 input_expression = when_expression 進(jìn)行計算。
返回第一個取值為 TRUE 的 (input_expression = when_expression) 的 result_expression。
如果沒有取值為 TRUE 的 input_expression = when_expression，則當(dāng)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(shù)：
按指定順序為每個 WHEN 子句的 Boolean_expression 求值。
返回第一個取值為 TRUE 的 Boolean_expression 的 result_expression。
如果沒有取值為 TRUE 的 Boolean_expression，則當(dāng)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。
下面分享一些mysql case when語句的例子。
A. 使用帶有簡單 CASE 函數(shù)的 SELECT 語句
在 SELECT 語句中，簡單 CASE 函數(shù)僅檢查是否相等，而不進(jìn)行其它比較。
例子，使用 CASE 函數(shù)更改圖書分類顯示。
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
注釋，后來我試了一下不讓用category=。
我使用的代碼為：
SELECT
case gender
WHEN 1 THEN 'NAN'
WHEN 0 THEN 'NV'
end as gender
FROM
t_swidy_day_nutrient
結(jié)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. 使用帶有簡單 CASE 函數(shù)和 CASE 搜索函數(shù)的
SELECT 語句
在 SELECT 語句中，CASE 搜索函數(shù)允許根據(jù)比較值在結(jié)果集內(nèi)對值進(jìn)行替換。
例子：根據(jù)圖書的價格范圍將價格（money 列）顯示為文本注釋。
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
結(jié)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. 使用帶有 SUBSTRING 和 SELECT 的 CASE 函數(shù)
例子，使用 CASE 和 THEN 生成一個有關(guān)作者、圖書標(biāo)識號和每個作者所著圖書類型的列表。
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
結(jié)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 中被誤用最多的關(guān)鍵字之一。
雖然，可能以前用過這個關(guān)鍵字來創(chuàng)建字段，但是它還具有更多用法。
例如，可以在 WHERE 子句中使用 CASE。
首先，來看下 CASE 的語法。在一般的 SELECT 中，其語法如下：
SELECT <myColumnSpec> =
CASE
WHEN <A> THEN <somethingA>
WHEN <B> THEN <somethingB>
ELSE <somethingE>
END
以上代碼，需要用具體的參數(shù)代替尖括號中的內(nèi)容。
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
這是 CASE 的典型用法，但是使用 CASE 其實可以做更多的事情。
比方說下面的 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
甚至還可以組合這些選項，添加一個 ORDER BY 子句，例如：
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，為了在 GROUP BY 塊中使用 CASE，查詢語句需要在 GROUP BY 塊中重復(fù) SELECT 塊中的 CASE 塊。
除了選擇自定義字段之外，在很多情況下 CASE 都非常有用。
稍加深入，還可以得到以前認(rèn)為不可能得到的分組排序結(jié)果集。
使用CASE WHEN進(jìn)行字符串替換處理/*
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
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(dá)式
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
/*
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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