mysql中case语句的作用与实例

/*创建表c1*/

CREATE TABLE c1 (
    id INT,
    sex VARCHAR (10),
    score INT
);

/*插入数据*/
INSERT INTO  c1
VALUES
    (1001, '男' ,65),
    (1002, '男' ,75),
    (1003, '女' ,82),
    (1004, '女' ,70),
    (1005, '男' ,89),
    (1006, '女' ,90),
    (1007, '男' ,85);

SELECT * FROM c1;

/*将性别男、女分别用0、1替换*/

SELECT
    id,
    CASE 
        WHEN sex = '男' THEN 0
        WHEN sex = '女' THEN 1 END sex ,
    score
FROM c1;

/*将成绩分为小于60 差，60~80 中，80~90良，90以上优*/

SELECT * ,
    CASE 
        WHEN score<60 THEN '差'
        WHEN score>=60 AND score<80 THEN '中'
        WHEN score>=80 AND score<90 THEN '良'
        WHEN score>=90  THEN '优' END level
FROM c1;

/*将成绩等级分为四列*/
SELECT id, sex,score,
    CASE  WHEN score<60 THEN '差' END AS level1,
    CASE  WHEN score>=60 AND score<80 THEN '中' END AS level2,
    CASE  WHEN score>=80 AND score<90 THEN '良' END AS level3,
    CASE  WHEN score>=90  THEN '优'END  AS level4
FROM c1;

/*按照性别计算各成绩等级中的数量*/

SELECT sex,
    SUM(CASE WHEN score<60 THEN 1 ELSE 0 END) AS '差',
    SUM(CASE WHEN score>=60 AND score<80 THEN 1 ELSE 0 END) AS '中',
    SUM(CASE WHEN score>=80 AND score<90 THEN 1 ELSE 0 END) AS '良',
    SUM(CASE WHEN score>=90 THEN 1 ELSE 0 END) AS '优'
FROM c1
GROUP BY sex;

原文链接：MySQL case