SQL 26计算25岁以上和以下的用户数量

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1、题目

现在运营想要将用户划分为25岁以下和25岁及以上两个年龄段,分别查看这两个年龄段用户数量

本题注意：age为null 也记为 25岁以下

示例：user_profile

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：

age_cut	number
25岁以下	4
25岁及以上	3

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：
25岁以下|4
25岁及以上|3
复制代码

2、思路🧠

问题分解：

Age is divided into two groups： "25岁及以上", "25岁以下"
统计用户数量：count,Each segment is counted separately,用 group by age_cut 分组
Of course this question can also be usedUNION ALL解决,用if更直观

解决方法：

UNION 组合查询（去重）,UNION ALL组合查询（不去重）
IF条件查询,IF(age>=25, "25岁及以上", "25岁以下")

CASE函数查询,是一种多分支的函数,可以根据条件列表的值返回多个可能的结果表达式中的一个;可用在任何允许使用表达式的地方,但不能单独作为一个语句执行.分为：简单CASE函数、搜索CASE函数

#简单CASE函数
CASE 测试表达式
WHEN 简单表达式1 THEN 结果表达式1
WHEN 简单表达式2 THEN 结果表达式2 ……
WHEN 简单表达式n THEN 结果表达式n
[ ELSE 结果表达式n+1 ]
END

#搜索CASE函数
CASE
WHEN 布尔表达式1 THEN 结果表达式1
WHEN 布尔表达式2 THEN 结果表达式2 ……
WHEN 布尔表达式n THEN 结果表达式n 
[ ELSE 结果表达式n+1 ]
END
复制代码

3、代码

commit AC

SELECT '25岁以下' as age_cut,COUNT(*)
FROM user_profile
WHERE age < 25 OR age is null 
UNION ALL
SELECT '25岁及以上' as age_cut,COUNT(*)
FROM user_profile
WHERE age >= 25

SELECT IF(age>=25,'25岁及以上','25岁以下') AS age_cut,COUNT(*)
FROM user_profile
GROUP BY age_cut

SELECT (case when age>=25 then '25岁及以上' else '25岁以下' end) AS age_cut,COUNT(*)
FROM user_profile
GROUP BY age_cut
复制代码

4、总结

Right on the topicSQLgrammar and basics,学会使用UNIONto perform a combined query,IF、CASEOther conditional queries will also be used,like inner join、外连接、左连接、There must be a relevant understanding of the right join and so on,Second when you write a lotSQL之后,Just learn to do itSQL的优化,This will greatly reduce the data query time.