id	device_id	gender	age	university	gpa	active_days_within_30	question_cnt	answer_cnt
1	2138	male	21	北京大学	3.4	7	2	12
2	3214	male		复旦大学	4	15	5	25
3	6543	female	20	北京大学	3.2	12	3	30
4	2315	female	23	浙江大学	3.6	5	1	2
5	5432	male	25	山东大学	3.8	20	15	70
6	2131	male	28	山东大学	3.3	15	7	13
7	4321	male	26	复旦大学	3.6	9	6	52

根据示例，你的查询应返回以下结果：

示例1

输入:

drop table if exists `user_profile`;
drop table if  exists `question_practice_detail`;
CREATE TABLE `user_profile` (
`id` int NOT NULL,
`device_id` int NOT NULL,
`gender` varchar(14) NOT NULL,
`age` int ,
`university` varchar(32) NOT NULL,
`gpa` float,
`active_days_within_30` int ,
`question_cnt` int ,
`answer_cnt` int 
);
CREATE TABLE `question_practice_detail` (
`id` int NOT NULL,
`device_id` int NOT NULL,
`question_id`int NOT NULL,
`result` varchar(32) NOT NULL
);
CREATE TABLE `question_detail` (
`id` int NOT NULL,
`question_id`int NOT NULL,
`difficult_level` varchar(32) NOT NULL
);

INSERT INTO user_profile VALUES(1,2138,'male',21,'北京大学',3.4,7,2,12);
INSERT INTO user_profile VALUES(2,3214,'male',null,'复旦大学',4.0,15,5,25);
INSERT INTO user_profile VALUES(3,6543,'female',20,'北京大学',3.2,12,3,30);
INSERT INTO user_profile VALUES(4,2315,'female',23,'浙江大学',3.6,5,1,2);
INSERT INTO user_profile VALUES(5,5432,'male',25,'山东大学',3.8,20,15,70);
INSERT INTO user_profile VALUES(6,2131,'male',28,'山东大学',3.3,15,7,13);
INSERT INTO user_profile VALUES(7,4321,'male',28,'复旦大学',3.6,9,6,52);
INSERT INTO question_practice_detail VALUES(1,2138,111,'wrong');
INSERT INTO question_practice_detail VALUES(2,3214,112,'wrong');
INSERT INTO question_practice_detail VALUES(3,3214,113,'wrong');
INSERT INTO question_practice_detail VALUES(4,6543,111,'right');
INSERT INTO question_practice_detail VALUES(5,2315,115,'right');
INSERT INTO question_practice_detail VALUES(6,2315,116,'right');
INSERT INTO question_practice_detail VALUES(7,2315,117,'wrong');
INSERT INTO question_practice_detail VALUES(8,5432,117,'wrong');
INSERT INTO question_practice_detail VALUES(9,5432,112,'wrong');
INSERT INTO question_practice_detail VALUES(10,2131,113,'right');
INSERT INTO question_practice_detail VALUES(11,5432,113,'wrong');
INSERT INTO question_practice_detail VALUES(12,2315,115,'right');
INSERT INTO question_practice_detail VALUES(13,2315,116,'right');
INSERT INTO question_practice_detail VALUES(14,2315,117,'wrong');
INSERT INTO question_practice_detail VALUES(15,5432,117,'wrong');
INSERT INTO question_practice_detail VALUES(16,5432,112,'wrong');
INSERT INTO question_practice_detail VALUES(17,2131,113,'right');
INSERT INTO question_practice_detail VALUES(18,5432,113,'wrong');
INSERT INTO question_practice_detail VALUES(19,2315,117,'wrong');
INSERT INTO question_practice_detail VALUES(20,5432,117,'wrong');
INSERT INTO question_practice_detail VALUES(21,5432,112,'wrong');
INSERT INTO question_practice_detail VALUES(22,2131,113,'right');
INSERT INTO question_practice_detail VALUES(23,5432,113,'wrong');
INSERT INTO question_detail VALUES(1,111,'hard');
INSERT INTO question_detail VALUES(2,112,'medium');
INSERT INTO question_detail VALUES(3,113,'easy');
INSERT INTO question_detail VALUES(4,115,'easy');
INSERT INTO question_detail VALUES(5,116,'medium');
INSERT INTO question_detail VALUES(6,117,'easy');

输出:

2138|male|20-24岁
3214|male|其他
6543|female|20-24岁
2315|female|20-24岁
5432|male|25岁及以上
2131|male|25岁及以上
4321|male|25岁及以上

原站题解

select device_id,gender,case when age<20 then '20岁以下'
when age>=20 and age<=24 then '20-24岁'
when age>=25 then '25岁及以上'
when age is null then '其他'
end as age_cut
from user_profile

select device_id,gender,
case 
when age<20 then '20岁以下'
when age <=24 and age>=20 then '20-24岁'
when age>=25 then '25岁及以上'
else '其他'
end as age_cut
from user_profile

select device_id,
       gender,
       case
        when age >= 25 then '25岁及以上'
        when age < 20 then '20岁以下'
        when 20 <= age < 25 then '20-24岁'
        else '其他' 
       end as age_cut
from user_profile

select device_id,gender,(case
  when age >= 20 and age <= 24 then '20-24岁'
  when age < 20 then '20岁以下'
  when age >= 25 then '25岁及以上'
  else '其他' end ) as age_cut
  from user_profile
  
  

# select device_id , gender , (case when age >= 20 and age <= 24 then '20-24岁' 
#         when age >= 25 then '25岁及以上' 
#         else '其他' end) age_cnt
# from user_profile


select device_id , gender , if( age >= 20 and age <= 24 , '20-24岁' ,
        if( age >= 25 , '25岁及以上' ,'其他' )) age_cnt
from user_profile