使用CASE WHEN进行字符串替换处理

/*
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;   

简单语句

SELECT CASE WHEN 10*2=30 THEN '30 correct'
   WHEN 10*2=40 THEN '40 correct'
   ELSE 'Should be 10*2=20'
END;
//不够60秒的算一分钟，
SELECT case MOD(70,60) when 0 then floor(70/60) ELSE floor(70/60)+1 end;

多重表达式

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

/*
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;

#表的创建
CREATE TABLE `lee` (
`id` int(10) NOT NULL AUTO_INCREMENT, 
`name` char(20) DEFAULT NULL, 
`birthday` datetime DEFAULT NULL, 
PRIMARY KEY (`id`)) ENGINE=InnoDB DEFAULT CHARSET=utf8

#数据插入：
insert into lee(name,birthday) values ('sam','1990-01-01');
insert into lee(name,birthday) values ('lee','1980-01-01');
insert into lee(name,birthday) values ('john','1985-01-01');


#使用case when语句
select name,
 case
        when birthday<'1981' then 'old'
        when birthday>'1988' then 'yong'
        else 'ok' END YORN
from lee;
 
select NAME,
 case name
     when 'sam' then 'yong'
        when 'lee' then 'handsome'
        else 'good' end
from lee;
#当然了case when语句还可以复合

select name,birthday,
 case
     when birthday>'1983' then 'yong'
        when name='lee' then 'handsome'
        else 'just so so ' end
from lee;

#在这里用sql语句进行日期比较的话，需要对年加引号。要不然可能结果可能和预期的结果会不同。我的mysql版本5.1

#当然也可以用year函数来实现，以第一个sql为例

select NAME,
 CASE
     when year(birthday)>1988 then 'yong'
        when year(birthday)<1980 then 'old'
        else 'ok' END
from lee;

create table penalties
(
 paymentno INTEGER not NULL,
    payment_date DATE not null,
    amount DECIMAL(7,2) not null,
    primary key(paymentno)
)

insert into penalties values(1,'2008-01-01',3.45);
insert into penalties values(2,'2009-01-01',50.45);
insert into penalties values(3,'2008-07-01',80.45);

#对罚款登记分为三类，第一类low，包括大于0小于等于40的罚款，第二类moderate大于40
#到80之间的罚款，第三类high包含所有大于80的罚款。
#统计出属于low的罚款编号。

#第一道题的解法与上面的相同
select paymentno,amount,
 case
     when amount>0 and amount<=40 then 'low'
        when amount>40 and amount<=80 then 'moderate'
        when amount>80 then 'high'
        else 'incorrect' end lvl
from `penalties`

#统计出属于low的罚款编号。重点看这里的解决方法
#方法1.
select paymentno,amount
from `penalties`
where case
 when amount>0 and  amount<=40 then 'low'
    when amount>40 and amount<=80 then 'moderate'
    when amount>80 then 'high'
    else 'incorrect' end ='low';

#方法2
select *
from (select paymentno,amount,
 case
     when amount>0 and amount<=40 then 'low'
        when amount>40 and amount<=80 then 'moderate'
        when amount>80 then 'high'
        else 'incorrect' end lvl
from `penalties`) as p
where p.lvl='low';