学习mysql的case when语句用法

作者: 祈祷23400441
来源: 51数据库
2020-09-01

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)

*/
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;