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Tutorial on the principle and application of database system (062) -- MySQL exercise questions: operation questions 32-38 (6)

2022-07-28 13:45:00 【Rsda DBA_ WGX】

Database system principle and Application Tutorial （062）—— MySQL Exercises ： Operation questions 32-38（ 6、 ... and ）

32、 Subquery

subject ： Query the minimum of each school gpa, Output results by university Ascending order .

Example ：user_profile The data in the table are as follows .

id	device_id	gender	age	university	gpa	active_days_within_30	question_cnt	answer_cnt
1	2138	male	21	Peking University,	3.4	7	2	12
2	3214	male		Fudan University	4	15	5	25
3	6543	female	20	Peking University,	3.2	12	3	30
4	2315	female	23	Zhejiang University	3.6	5	1	2
5	5432	male	25	Shandong University	3.8	20	15	70
6	2131	male	28	Shandong University	3.3	15	7	13
7	4321	female	26	Fudan University	3.6	9	6	52

The query should return the following results ：

device_id	university	gpa
6543	Peking University,	3.2000
4321	Fudan University	3.6000
2131	Shandong University	3.3000
2315	Zhejiang University	3.6000

Table structure and data are as follows ：

/* drop table if exists user_profile; 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 ); INSERT INTO user_profile VALUES(1,2138,'male',21,' Peking University, ',3.4,7,2,12); INSERT INTO user_profile VALUES(2,3214,'male',null,' Fudan University ',4.0,15,5,25); INSERT INTO user_profile VALUES(3,6543,'female',20,' Peking University, ',3.2,12,3,30); INSERT INTO user_profile VALUES(4,2315,'female',23,' Zhejiang University ',3.6,5,1,2); INSERT INTO user_profile VALUES(5,5432,'male',25,' Shandong University ',3.8,20,15,70); INSERT INTO user_profile VALUES(6,2131,'male',28,' Shandong University ',3.3,15,7,13); INSERT INTO user_profile VALUES(7,4321,'male',28,' Fudan University ',3.6,9,6,52); */

answer ：

/* select a.device_id, b.university, round(b.gpa,4) gpa from user_profile a join (select university, min(gpa) gpa from user_profile group by university) b on a.gpa = b.gpa and a.university = b.university order by a.university; */
mysql> select a.device_id, b.university, round(b.gpa,4) gpa
    -> from user_profile a join 
    ->        (select university, min(gpa) gpa
    ->         from user_profile group by university) b
    ->         on a.gpa = b.gpa and a.university = b.university
    -> order by a.university;
+-----------+--------------+--------+
| device_id | university   | gpa    |
+-----------+--------------+--------+
|      6543 |  Peking University,      | 3.2000 |
|      4321 |  Fudan University      | 3.6000 |
|      2131 |  Shandong University      | 3.3000 |
|      2315 |  Zhejiang University      | 3.6000 |
+-----------+--------------+--------+
4 rows in set (0.00 sec)

33、 Group query and statistics （1）

subject ： Query every user of Fudan University in 8 The number of questions practiced in the month and the number of questions answered correctly , Take out the corresponding detailed data . For in 8 Users who haven't practiced in this month , Answer number result return 0.

Example ：user_profile The data in the table are as follows .

id	device_id	gender	age	university	gpa	active_days_within_30
1	2138	male	21	Peking University,	3.4	7
2	3214	male		Fudan University	4.0	15
3	6543	female	20	Peking University,	3.2	12
4	2315	female	23	Zhejiang University	3.6	5
5	5432	male	25	Shandong University	3.8	20
6	2131	male	28	Shandong University	3.3	15
7	4321	female	26	Fudan University	3.6	9

Example ：question_practice_detail The data in the table are as follows .

id	device_id	question_id	result	date
1	2138	111	wrong	2021-05-03
2	3214	112	wrong	2021-05-09
3	3214	113	wrong	2021-06-15
4	6543	111	right	2021-08-13
5	2315	115	right	2021-08-13
6	2315	116	right	2021-08-14
7	2315	117	wrong	2021-08-15
……

The query should return the following results ：

device_id	university	question_cnt	right_question_cnt
3214	Fudan University	3	0
4321	Fudan University	0	0

Table structure and data are as follows ：

/* 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 ); CREATE TABLE `question_practice_detail` ( `id` int NOT NULL, `device_id` int NOT NULL, `question_id`int NOT NULL, `result` varchar(32) NOT NULL, `date` date NOT NULL ); INSERT INTO user_profile VALUES(1,2138,'male',21,' Peking University, ',3.4,7); INSERT INTO user_profile VALUES(2,3214,'male',null,' Fudan University ',4.0,15); INSERT INTO user_profile VALUES(3,6543,'female',20,' Peking University, ',3.2,12); INSERT INTO user_profile VALUES(4,2315,'female',23,' Zhejiang University ',3.6,5); INSERT INTO user_profile VALUES(5,5432,'male',25,' Shandong University ',3.8,20); INSERT INTO user_profile VALUES(6,2131,'male',28,' Shandong University ',3.3,15); INSERT INTO user_profile VALUES(7,4321,'male',28,' Fudan University ',3.6,9); INSERT INTO question_practice_detail VALUES(1,2138,111,'wrong','2021-05-03'); INSERT INTO question_practice_detail VALUES(2,3214,112,'wrong','2021-05-09'); INSERT INTO question_practice_detail VALUES(3,3214,113,'wrong','2021-06-15'); INSERT INTO question_practice_detail VALUES(4,6543,111,'right','2021-08-13'); INSERT INTO question_practice_detail VALUES(5,2315,115,'right','2021-08-13'); INSERT INTO question_practice_detail VALUES(6,2315,116,'right','2021-08-14'); INSERT INTO question_practice_detail VALUES(7,2315,117,'wrong','2021-08-15'); INSERT INTO question_practice_detail VALUES(8,3214,112,'wrong','2021-05-09'); INSERT INTO question_practice_detail VALUES(9,3214,113,'wrong','2021-08-15'); INSERT INTO question_practice_detail VALUES(10,6543,111,'right','2021-08-13'); INSERT INTO question_practice_detail VALUES(11,2315,115,'right','2021-08-13'); INSERT INTO question_practice_detail VALUES(12,2315,116,'right','2021-08-14'); INSERT INTO question_practice_detail VALUES(13,2315,117,'wrong','2021-08-15'); INSERT INTO question_practice_detail VALUES(14,3214,112,'wrong','2021-08-16'); INSERT INTO question_practice_detail VALUES(15,3214,113,'wrong','2021-08-18'); INSERT INTO question_practice_detail VALUES(16,6543,111,'right','2021-08-13'); */

answer ：

/* select u.device_id, u.university, count(question_id) question_cnt, sum(if(q.result = 'right', 1, 0)) right_question_cnt from user_profile u left join question_practice_detail q on u.device_id = q.device_id where u.university = ' Fudan University ' and (month(q.date) = 8 or month(q.date) is null) group by u.device_id, u.university; */
mysql> select u.device_id, u.university,
    ->        count(question_id) question_cnt, 
    ->        sum(if(q.result = 'right', 1, 0)) right_question_cnt 
    -> from user_profile u left join question_practice_detail q
    -> on u.device_id = q.device_id
    -> where u.university = ' Fudan University ' and (month(q.date) = 8 or month(q.date) is null)
    -> group by u.device_id, u.university;
+-----------+--------------+--------------+--------------------+
| device_id | university   | question_cnt | right_question_cnt |
+-----------+--------------+--------------+--------------------+
|      3214 |  Fudan University      |            3 |                  0 |
|      4321 |  Fudan University      |            0 |                  0 |
+-----------+--------------+--------------+--------------------+
2 rows in set (0.00 sec)

/*
  explain ：
（1）count(question_id)： Not available here  count(*), because  count(*)  Statistics are all records , And of course  question_id  Empty record ,count(question_id)  Only statistics  question_id  Not empty record 
（2）where  Conditions must be specified ：month(q.date) is null, Otherwise, even if you use  left join, Records that do not meet the connection conditions will also be filtered

34、 Group query and statistics （2）

subject ： Query the accuracy of users of Zhejiang University in answering different difficult questions , Take out the corresponding data and output it in ascending order of accuracy .

Example ： user_profile The data in the table are as follows .

id	device_id	gender	age	university	gpa	active_days_within_30	question_cnt	answer_cnt
1	2138	male	21	Peking University,	3.4	7	2	12
2	3214	male		Fudan University	4	15	5	25
3	6543	female	20	Peking University,	3.2	12	3	30
4	2315	female	23	Zhejiang University	3.6	5	1	2
5	5432	male	25	Shandong University	3.8	20	15	70
6	2131	male	28	Shandong University	3.3	15	7	13
7	4321	female	26	Fudan University	3.6	9	6	52

Example ： question_practice_detail The data in the table are as follows .

id	device_id	question_id	result
1	2138	111	wrong
2	3214	112	wrong
3	3214	113	wrong
4	6543	111	right
5	2315	115	right
6	2315	116	right
7	2315	117	wrong

Example ： question_detail The data in the table are as follows .

question_id	difficult_level
111	hard
112	medium
113	easy
115	easy
116	medium
117	easy

The query should return the following results ：

difficult_level	correct_rate
easy	0.5000
medium	1.0000

Table structure and data are as follows ：

/* 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, `date` date 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,' Peking University, ',3.4,7,2,12); INSERT INTO user_profile VALUES(2,3214,'male',null,' Fudan University ',4.0,15,5,25); INSERT INTO user_profile VALUES(3,6543,'female',20,' Peking University, ',3.2,12,3,30); INSERT INTO user_profile VALUES(4,2315,'female',23,' Zhejiang University ',3.6,5,1,2); INSERT INTO user_profile VALUES(5,5432,'male',25,' Shandong University ',3.8,20,15,70); INSERT INTO user_profile VALUES(6,2131,'male',28,' Shandong University ',3.3,15,7,13); INSERT INTO user_profile VALUES(7,4321,'male',28,' Fudan University ',3.6,9,6,52); INSERT INTO question_practice_detail VALUES(1,2138,111,'wrong','2021-05-03'); INSERT INTO question_practice_detail VALUES(2,3214,112,'wrong','2021-05-09'); INSERT INTO question_practice_detail VALUES(3,3214,113,'wrong','2021-06-15'); INSERT INTO question_practice_detail VALUES(4,6543,111,'right','2021-08-13'); INSERT INTO question_practice_detail VALUES(5,2315,115,'right','2021-08-13'); INSERT INTO question_practice_detail VALUES(6,2315,116,'right','2021-08-14'); INSERT INTO question_practice_detail VALUES(7,2315,117,'wrong','2021-08-15'); INSERT INTO question_practice_detail VALUES(8,3214,112,'wrong','2021-05-09'); INSERT INTO question_practice_detail VALUES(9,3214,113,'wrong','2021-08-15'); INSERT INTO question_practice_detail VALUES(10,6543,111,'right','2021-08-13'); INSERT INTO question_practice_detail VALUES(11,2315,115,'right','2021-08-13'); INSERT INTO question_practice_detail VALUES(12,2315,116,'right','2021-08-14'); INSERT INTO question_practice_detail VALUES(13,2315,117,'wrong','2021-08-15'); INSERT INTO question_practice_detail VALUES(14,3214,112,'wrong','2021-08-16'); INSERT INTO question_practice_detail VALUES(15,3214,113,'wrong','2021-08-18'); INSERT INTO question_practice_detail VALUES(16,6543,111,'right','2021-08-13'); 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'); */

answer ：

/* select b.difficult_level, round(sum(if(result = 'right', 1, 0))/count(*), 4) correct_rate from question_practice_detail a join question_detail b on a.question_id = b.question_id where a.device_id in (select device_id from user_profile where university = ' Zhejiang University ') group by b.difficult_level order by correct_rate; */
mysql> select b.difficult_level,
    ->        round(sum(if(result = 'right', 1, 0))/count(*), 4) correct_rate
    -> from question_practice_detail a join question_detail b
    -> on a.question_id = b.question_id
    -> where a.device_id in (select device_id from user_profile where university = ' Zhejiang University ')
    -> group by b.difficult_level
    -> order by correct_rate;
+-----------------+--------------+
| difficult_level | correct_rate |
+-----------------+--------------+
| easy            |       0.5000 |
| medium          |       1.0000 |
+-----------------+--------------+
2 rows in set (0.00 sec)

35、 Sort query results （1）

subject ： Query the user age in the user information table , Take out the corresponding data and sort them in ascending order of age .

Example ：user_profile The data in the table are as follows .

id	device_id	gender	age	university	gpa
1	2138	male	21	Peking University,	3.4
2	3214	male	23	Fudan University	4
3	6543	female	20	Peking University,	3.2
4	2315	female	23	Zhejiang University	3.6
5	5432	male	25	Shandong University	3.8
6	2131	male	28	Beijing Normal University	3.3

The query should return the following results ：

device_id	age
6534	20
2138	21
3214	23
2315	23
5432	25
2131	28

Table structure and data are as follows ：

/* drop table if exists user_profile; 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); INSERT INTO user_profile VALUES(1,2138,'male',21,' Peking University, ',3.4); INSERT INTO user_profile VALUES(2,3214,'male',23,' Fudan University ',4.0); INSERT INTO user_profile VALUES(3,6543,'female',20,' Peking University, ',3.2); INSERT INTO user_profile VALUES(4,2315,'female',23,' Zhejiang University ',3.6); INSERT INTO user_profile VALUES(5,5432,'male',25,' Shandong University ',3.8); INSERT INTO user_profile VALUES(6,2131,'male',28,' Beijing Normal University ',3.3); */

answer ：

mysql> select device_id, age from user_profile order by age;
+-----------+------+
| device_id | age  |
+-----------+------+
|      6543 |   20 |
|      2138 |   21 |
|      3214 |   23 |
|      2315 |   23 |
|      5432 |   25 |
|      2131 |   28 |
+-----------+------+
6 rows in set (0.00 sec)

36、 Sort query results （1）

subject ： Query the age and... In the user information table gpa data , And first according to gpa Ascending sort , Then sort the output in ascending order of age .

Example ：user_profile The data in the table are as follows .

id	device_id	gender	age	university	gpa
1	2138	male	21	Peking University,	3.4
2	3214	male	23	Fudan University	4
3	6543	female	20	Peking University,	3.2
4	2315	female	23	Zhejiang University	3.6
5	5432	male	25	Shandong University	3.8
6	2131	male	28	Beijing Normal University	3.3

The query should return the following results ：

device_id	gpa	age
6543	3.2	20
2131	3.3	28
2138	3.4	21
2315	3.6	23
5432	3.8	25
3214	4	23

answer ：

mysql> select device_id, gpa, age from user_profile order by gpa, age;
+-----------+------+------+
| device_id | gpa  | age  |
+-----------+------+------+
|      6543 |  3.2 |   20 |
|      2131 |  3.3 |   28 |
|      2138 |  3.4 |   21 |
|      2315 |  3.6 |   23 |
|      5432 |  3.8 |   25 |
|      3214 |    4 |   23 |
+-----------+------+------+
6 rows in set (0.00 sec)

37、 Sort query results （3）

subject ： Query the corresponding data in the user information table , And first according to gpa Descending 、 Sort the output in descending order of age .

Example ：user_profile The data in the table are as follows .

id	device_id	gender	age	university	gpa
1	2138	male	21	Peking University,	3.4
2	3214	male	23	Fudan University	4
3	6543	female	20	Peking University,	3.2
4	2315	female	23	Zhejiang University	3.6
5	5432	male	25	Shandong University	3.8
6	2131	male	28	Beijing Normal University	3.3

The query should return the following results ：

device_id	gpa	age
3214	4	23
5432	3.8	25
2315	3.6	23
2138	3.4	21
2131	3.3	28
6543	3.2	20

answer ：

mysql> select device_id, gpa, age from user_profile order by gpa desc, age desc;
+-----------+------+------+
| device_id | gpa  | age  |
+-----------+------+------+
|      3214 |    4 |   23 |
|      5432 |  3.8 |   25 |
|      2315 |  3.6 |   23 |
|      2138 |  3.4 |   21 |
|      2131 |  3.3 |   28 |
|      6543 |  3.2 |   20 |
+-----------+------+------+
6 rows in set (0.00 sec)

38、 The use of aggregate functions

subject ： Inquire about 2021 year 8 The total number of users who have practiced the topic and the total number of times they have practiced the topic in the month , Take out the corresponding results .

Example ：question_practice_detail The data in the table are as follows .

id	device_id	question_id	result	date
1	2138	111	wrong	2021-05-03
2	3214	112	wrong	2021-05-09
3	3214	113	wrong	2021-06-15
4	6543	111	right	2021-08-13
5	2315	115	right	2021-08-13
6	2315	116	right	2021-08-14
7	2315	117	wrong	2021-08-15
……

The query should return the following results ：

did_cnt	question_cnt
3	12

answer ：

/* select count(distinct device_id) did_cnt, count(*) question_cnt from question_practice_detail where date between '2021-8-1' and '2021-8-31'; */
mysql> select count(distinct device_id) did_cnt,
    ->        count(*) question_cnt
    -> from question_practice_detail
    -> where date between '2021-8-1' and '2021-8-31';
+---------+--------------+
| did_cnt | question_cnt |
+---------+--------------+
|       3 |           12 |
+---------+--------------+
1 row in set (0.02 sec)

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Tutorial on the principle and application of database system (062) -- MySQL exercise questions: operation questions 32-38 (6)

Database system principle and Application Tutorial （062）—— MySQL Exercises ： Operation questions 32-38（ 6、 ... and ）

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