【SQL中阶】 SQL多表联查（配合多条件复杂查询）

using

类似两表关联的查询条件例如 using(id) 等价于 join on a.id=b.id

select
    v.vend_id  vend_id,count(p.prod_id) prod_id
from
    Vendors v
    left join Products p on p.vend_id = v.vend_id
group by
    vend_id
order by
    vend_id
*************************************************
select
    v.vend_id  vend_id,count(p.prod_id) prod_id
from
    Vendors v
    left join Products p using(vend_id)
group by
    vend_id
order by
    vend_id

【案例具体所需要的表和SQL语句在文章末尾】

最难的点在于 平均答题数目 SQL函数如下（使用函数较多 关键字多 逻辑较为复杂）

round(
        (count(q.question_id) / count(distinct q.device_id)),4
    ) avg_answer_cnt

查找每个学校用户的平均答题数目(说明：某学校用户平均答题数量计算方式为该学校用户答题总次数除以答过题的不同用户个数)根据示例，你的查询应返回以下结果（结果保留4位小数），注意：结果按照university升序排序

select
    u.university,
    round(
        (count(q.question_id) / count(distinct q.device_id)),4
    ) avg_answer_cnt
from
    user_profile u
    left join question_practice_detail q 
    on q.device_id = u.device_id
group by
    u.university
order by
    u.university

计算不同学校、不同难度的用户平均答题量，根据示例，你的查询应返回以下结果(结果在小数点位数保留4位，4位之后四舍五入)

select
    u.university,
    qu.difficult_level, 
    round(
        (count(q.question_id) / count(distinct q.device_id)),4
    ) avg_answer_cnt
from
    user_profile u
    join question_practice_detail q on u.device_id = q.device_id
    JOIN question_detail qu on q.question_id = qu.question_id
group by
    qu.difficult_level,u.university

计算山东、不同难度的用户平均答题量，根据示例，你的查询应返回以下结果(结果在小数点位数保留4位，4位之后四舍五入)

SELECT
    u.university,
    qu.difficult_level, 
    round(
        (count(q.question_id) / count(distinct q.device_id)),4
    ) avg_answer_cnt
from
    user_profile u
    join question_practice_detail q on u.device_id = q.device_id
    JOIN question_detail qu on q.question_id = qu.question_id
where u.university = '山东大学'
group by qu.difficult_level

案例所需要的表的的SQL语句如下：

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');