SQL 三個表聯接的join寫法---可用using(字段名)替換on t1.字段名=t2.字段名

 注意：left join等進行表了解時，要注意on字段的一對一，一對多，多對多的情況

           left join連接后的表字段出現null時的處理，若時count字段里出現要用ifnull(表達式，value)函數置零

Join連接的關系圖

 

 

select 
    university,
    difficult_level,
    count(qpd.question_id) / count(distinct qpd.device_id) 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

 

數據：統計每個學校各難度的用戶平均刷題數

關鍵字：每個，各，不同，用group by

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,'北京大學',3.4,7,2,12);
INSERT INTO user_profile VALUES(2,3214,'male',null,'復旦大學',4.0,15,5,25);
INSERT INTO user_profile VALUES(3,6543,'female',20,'北京大學',3.2,12,3,30);
INSERT INTO user_profile VALUES(4,2315,'female',23,'浙江大學',3.6,5,1,2);
INSERT INTO user_profile VALUES(5,5432,'male',25,'山東大學',3.8,20,15,70);
INSERT INTO user_profile VALUES(6,2131,'male',28,'山東大學',3.3,15,7,13);
INSERT INTO user_profile VALUES(7,4321,'male',28,'復旦大學',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');