mysql汇总的case关键词用法

CREATE TABLE `student` (
  `stu_id` int(20) NOT NULL AUTO_INCREMENT,
  `stu_name` varchar(50) DEFAULT NULL COMMENT ‘学生姓名‘,
  `stu_age` int(3) DEFAULT NULL COMMENT ‘学生年龄‘,
  `stu_sex` varchar(6) DEFAULT NULL COMMENT ‘学生性别‘,
  `add_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT ‘创建时间‘,
  PRIMARY KEY (`stu_id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8 COMMENT=‘学生表‘;

BEGIN;
INSERT INTO `student` VALUES (1, ‘Drew‘, 21, ‘男‘, null), (2, ‘Allen‘, 22, ‘男‘, null), (3, ‘King‘, 22, ‘男‘, null);
COMMIT;

-- ----------------------------
--      mysql中强大case语句
-- ----------------------------
SELECT
    CASE
        WHEN stu_age > 21 AND stu_age < 23 THEN 1000
        WHEN stu_age >= 23 THEN 2000
        ELSE stu_age 
    END stu_age
FROM student;

SELECT
    stu_id,
    stu_name,
    CASE
        WHEN stu_age = 21 THEN ‘二十一岁‘
        WHEN stu_age = 22 THEN ‘二十二岁‘
        WHEN stu_age = 23 THEN ‘二十三岁‘
        ELSE stu_age 
    END stu_age,
    stu_sex
FROM student;