【SQL基础】多表查询：子查询、连接查询（JOIN）、组合查询（UNION集合运算）

【SQL基础】多表查询：子查询、连接查询（JOIN）、组合查询（UNION集合运算）

〇、概述

1、内容

JOIN表连接（内连接INNER JOIN/JOIN）（外连接LEFT/RIGHT (OUTER) JOIN）

集合运算-UNION联合

2、建表语句

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);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(8,5432,118,'wrong');INSERT INTO question_practice_detail VALUES(9,5432,112,'wrong');INSERT INTO question_practice_detail VALUES(10,2131,114,'right');INSERT INTO question_practice_detail VALUES(11,5432,113,'wrong');

一、子查询

1、浙江大学用户题目回答情况

子查询实现：

SELECT device_id, question_id, resultFROM question_practice_detailWHERE device_id IN (SELECT device_id FROM user_profile WHERE university='浙江大学');

普通查询：

SELECT a.device_id, question_id, resultFROM question_practice_detail a, user_profile bWHERE a.device_id=b.device_idand b.university='浙江大学'ORDER BY question_id ASC;

二、连接查询【尝试使用表连接操作】

1、统计每个学校的答过题的用户的平均答题数

内连接方式（JOIN/INNER JOIN）：

SELECT university, --注意，如果两个表都有的字段，请带着表名 ROUND(COUNT(question_id)/COUNT(DISTINCT(question_practice_detail.device_id)),4) AS avg_answer_cntFROM user_profileJOIN question_practice_detailON user_profile.device_id=question_practice_detail.device_idGROUP BY universityORDER BY university ASC;

多表查询方式：

SELECT university, ROUND(COUNT(b.question_id)/COUNT(DISTINCT(b.device_id)),4) AS avg_answer_cntFROM user_profile a, question_practice_detail bWHERE a.device_id=b.device_idGROUP BY university;

2、统计每个学校各难度的用户平均刷题数

内连接方式：

SELECT university, difficult_level, ROUND(COUNT(b.question_id)/COUNT(DISTINCT(b.device_id)),4) AS avg_answer_cntFROM user_profile aINNER JOIN question_practice_detail bON a.device_id=b.device_idINNER JOIN question_detail cON c.question_id=b.question_idGROUP BY university,difficult_level;

多表查询方式：

SELECT university, difficult_level, ROUND(COUNT(b.question_id)/COUNT(DISTINCT(b.device_id)),4) AS avg_answer_cntFROM user_profile a, question_practice_detail b, question_detail cWHERE a.device_id=b.device_id and b.question_id=c.question_idGROUP BY university, difficult_level;

3、统计每个用户的平均刷题数

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

内连接方式：

SELECT university, difficult_level, ROUND(COUNT(b.question_id)/COUNT(DISTINCT(b.device_id)),4) AS avg_answer_cntFROM user_profile aINNER JOIN question_practice_detail bON a.device_id=b.device_idINNER JOIN question_detail cON b.question_id=c.question_idWHERE university='山东大学'GROUP BY difficult_level;

多表查询方式：

SELECT university, difficult_level, ROUND(COUNT(b.question_id)/COUNT(DISTINCT(b.device_id)),4) AS avg_answer_cntFROM user_profile a, question_practice_detail b, question_detail cWHERE a.device_id=b.device_id and b.question_id=c.question_id and university='山东大学'GROUP BY difficult_level;

三、组合查询

1、查找山东大学或者性别为男生的信息

（注意输出的顺序，先输出学校为山东大学再输出性别为男生的信息）

错误做法：

SELECT device_id, gender, age, gpaFROM user_profileWHERE university='山东大学' or gender='male';

正确做法：【不去重用union】

SELECT device_id, gender, age, gpaFROM user_profileWHERE university='山东大学'UNION ALLSELECT device_id, gender, age, gpaFROM user_profileWHERE gender='male';

作者：​​哥们要飞​​​​

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