MySQL 的CASE WHEN 语句使用说明

答案：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 易做图.' 
-> 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 易做图. | 
| 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 易做图.' 
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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