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一、多表查询
1、链接查询
1.1、SQL23 统计每个学校各难度的用户平均刷题数
- 描述
题目：运营想要计算一些参加了答题的不同学校、不同难度的用户平均答题量，请你写SQL取出相应数据
- 示例1
输入：
drop table if existsuser_profile;
drop table if existsquestion_practice_detail;
drop table if existsquestion_detail;
CREATE TABLEuser_profile(
idint NOT NULL,
device_idint NOT NULL,
gendervarchar(14) NOT NULL,
ageint ,
universityvarchar(32) NOT NULL,
gpafloat,
active_days_within_30int ,
question_cntint ,
answer_cntint
);
CREATE TABLEquestion_practice_detail(
idint NOT NULL,
device_idint NOT NULL,
question_idint NOT NULL,
resultvarchar(32) NOT NULL
);
CREATE TABLEquestion_detail(
idint NOT NULL,
question_idint NOT NULL,
difficult_levelvarchar(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：
北京大学|hard|1.0000
复旦大学|easy|1.0000
复旦大学|medium|1.0000
山东大学|easy|4.5000
山东大学|medium|3.0000
浙江大学|easy|5.0000
浙江大学|medium|2.0000
输入：
drop table if exists `user_profile`;
drop table if exists `question_practice_detail`;
drop table if exists `question_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：
北京大学|hard|1.0000
复旦大学|easy|1.0000
复旦大学|medium|1.0000
山东大学|easy|4.5000
山东大学|medium|3.0000
浙江大学|easy|5.0000
浙江大学|medium|2.0000
- SQL语句
select
university,
difficult_level,
round(
count(qpd.question_id) / count(distinct qpd.device_id),
4
) as avg_answer_cnt
from
question_practice_detail as qpd
left join user_profile as up on up.device_id = qpd.device_id
left join question_detail as qd on qd.question_id = qpd.question_id
group by
university,
difficult_level
select
university,
difficult_level,
round(
count(qpd.question_id) / count(distinct qpd.device_id),
4
) as avg_answer_cnt
from
question_practice_detail as qpd
left join user_profile as up on up.device_id = qpd.device_id
left join question_detail as qd on qd.question_id = qpd.question_id
group by
university,
difficult_level
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