MySQL的CASE WHEN語句的幾個使用實例

2024-07-24 13:05:48

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使用CASE WHEN進行字符串替換處理

復制代碼代碼如下:

mysql> select * from sales;

9 rows in set (0.01 sec)

SELECT name AS Name,
    CASE category
    WHEN "Holiday" THEN "Seasonal"
    WHEN "Profession" THEN "Bi_annual"
    WHEN "Literary" THEN "Random" END AS "Pattern"
FROM sales;

9 rows in set (0.00 sec)

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)
) ENGINE=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');

簡單語句

復制代碼代碼如下:

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

多重表達式

復制代碼代碼如下:

SELECT CASE 10*2
   WHEN 20 THEN '20 correct'
   WHEN 30 THEN '30 correct'
   WHEN 40 THEN '40 correct'
   ELSE 'No results'
END AS `result`;

在SELECT查詢中使用CASE WHEN

復制代碼代碼如下:

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;