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 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
