MySQL case when 用法

Case具有两种格式。简单Case函数和Case搜索函数。 

1、Case简单函数

CASE sex 
         WHEN '1' THEN '男' 
         WHEN '2' THEN '女' 
ELSE '其他' END 

2、Case搜索函数 

CASE WHEN sex = '1' THEN '男' 
         WHEN sex = '2' THEN '女' 
ELSE '其他' END 

下面用一些实例学习case：

存在Table：

/*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;

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;

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;