【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,
    result
FROM question_practice_detail
WHERE 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,
    result
FROM question_practice_detail a, user_profile b
WHERE 
    a.device_id=b.device_id
and
    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_cnt
FROM user_profile
JOIN question_practice_detail
ON user_profile.device_id=question_practice_detail.device_id
GROUP BY university
ORDER 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_cnt
FROM user_profile a, question_practice_detail b
WHERE a.device_id=b.device_id
GROUP 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_cnt
FROM user_profile a
INNER JOIN question_practice_detail b
ON a.device_id=b.device_id
INNER JOIN question_detail c
ON c.question_id=b.question_id
GROUP 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_cnt
FROM 
    user_profile a,
    question_practice_detail b,
    question_detail c
WHERE
    a.device_id=b.device_id
    and
    b.question_id=c.question_id
GROUP 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_cnt
FROM user_profile a
INNER JOIN question_practice_detail b
ON a.device_id=b.device_id
INNER JOIN question_detail c
ON b.question_id=c.question_id
WHERE 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_cnt
FROM
    user_profile a,
    question_practice_detail b,
    question_detail c
WHERE 
    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,
    gpa
FROM user_profile
WHERE
    university='山东大学'
    or
    gender='male';

正确做法：【不去重用union】

SELECT
    device_id,
    gender,
    age,
    gpa
FROM user_profile
WHERE university='山东大学'
UNION ALL
SELECT
    device_id,
    gender,
    age,
    gpa
FROM user_profile
WHERE gender='male';