一个在牛客上面刷题遇到的SQL问题。（原码是Mysql，我给换成SQL server了）

Question

题目我使用图片的格式放出来了

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');



---这部分是我写,运行不了
select university,difficult_level,count(qpd.question_id) / count(distinct qpd.device_id) as avg_answer_cnt
from user_profile as up
LEFT join question_practice_detail as qpd 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

--下面的是答案的，能运行成功！
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

Answer 1

select university,difficult_level,count(qpd.question_id) / count(distinct(qpd.device_id)) as avg_answer_cnt
from user_profile as up
join question_practice_detail as qpd on up.device_id = qpd.device_id
join question_detail as qd on qd.question_id = qpd.question_id
group by university,difficult_level

草，我自己给找到了，把左联结给删了，居然就运行成功了。