一、多表查询
1、链接查询
1.1、SQL24 统计每个用户的平均刷题数
描述
题目：运营想要查看参加了答题的山东大学的用户在不同难度下的平均答题题目数，请取出相应数据
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： 山东大学|easy|4.5000 山东大学|medium|3.0000
select u.university, q2.difficult_level, round( count(q2.question_id) / count(distinct q1.question_id), 4 ) avg_answer_cnt from user_profile u, question_practice_detail q1, question_detail q2 where u.device_id = q1.device_id and q1.question_id = q2.question_id and u.university = '山东大学' group by q2.difficult_level
