20200715 - 数据蛙第九期 - 1
学员作业
大板揪https://www.jianshu.com/p/a4da468ce484
喂!嘿嘿！https://www.jianshu.com/p/8a263f700926
想哭的我戴上喵的面具https://www.jianshu.com/p/6a71e2c3d49a
徐羽丰https://www.jianshu.com/p/0455e6551a69
Autumnhttps://www.jianshu.com/p/52e7b1086ba3
位春燕https://www.jianshu.com/p/c7ac7ed0e652
mysql基础
1、如何暂停或开启mysql服务？
# 前提：有net命令
net stop mysql # 暂停mysql
net start mysql # 启动mysql
通过 控制面板的 服务进行暂停
image-20200719144331902
步骤 2)：弹出“计算机管理”对话框，双击“服务和应用程序”，用户可查看计算机的服务状态，MySQL 的状态为“正在运行”，表明该服务已经启动，如图所示。
image-20200715223419145
注意：由于设置了 MySQL 为==自动启动==，在这里可以看到，服务已经启动，而且启动类型为自动。如果没有“正在运行”字样，说明 MySQL 服务未启动。可以直接在“计算机管理”窗口用菜单命令启动，也可以通过 DOS 命令启动 MySQL 服务。单击“开始”→“运行”，输入 cmd 命令，按回车键，弹出命令提示符界面，输入 net start mysql，按回车键，就能启动 MySQL 服务，停止服务的命令为 net stop mysql。
2、如何用cmd命令登录mysql窗口
C:\Users\Administrator>mysql -u root -p
Enter password: ******
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 13
Server version: 5.7.17-log MySQL Community Server (GPL)
Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
3、登录时mysql用到 -u \ -p 等参数，如何查看所有参数意思？
mysql --help
mysql Ver 14.14 Distrib 5.7.17, for Win64 (x86_64)
Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Usage: mysql [OPTIONS] [database]
-?, --help Display this help and exit.
-I, --help Synonym for -?
--auto-rehash Enable automatic rehashing. One doesn't need to use
'rehash' to get table and field completion, but startup
and reconnecting may take a longer time. Disable with
--disable-auto-rehash.
(Defaults to on; use --skip-auto-rehash to disable.)
-A, --no-auto-rehash
No automatic rehashing. One has to use 'rehash' to get
table and field completion. This gives a quicker start of
mysql and disables rehashing on reconnect.
--auto-vertical-output
Automatically switch to vertical output mode if the
result is wider than the terminal width.
-B, --batch Don't use history file. Disable interactive behavior.
(Enables --silent.)
--bind-address=name IP address to bind to.
--character-sets-dir=name
Directory for character set files.
--column-type-info Display column type information.
-c, --comments Preserve comments. Send comments to the server. The
default is --skip-comments (discard comments), enable
with --comments.
-C, --compress Use compression in server/client protocol.
-#, --debug[=#] This is a non-debug version. Catch this and exit.
--debug-check This is a non-debug version. Catch this and exit.
-T, --debug-info This is a non-debug version. Catch this and exit.
-D, --database=name Database to use.
--default-character-set=name
Set the default character set.
--delimiter=name Delimiter to be used.
--enable-cleartext-plugin
Enable/disable the clear text authentication plugin.
-e, --execute=name Execute command and quit. (Disables --force and history
file.)
-E, --vertical Print the output of a query (rows) vertically.
-f, --force Continue even if we get an SQL error.
--histignore=name A colon-separated list of patterns to keep statements
from getting logged into syslog and mysql history.
-G, --named-commands
Enable named commands. Named commands mean this program's
internal commands; see mysql> help . When enabled, the
named commands can be used from any line of the query,
otherwise only from the first line, before an enter.
Disable with --disable-named-commands. This option is
disabled by default.
-i, --ignore-spaces Ignore space after function names.
--init-command=name SQL Command to execute when connecting to MySQL server.
Will automatically be re-executed when reconnecting.
--local-infile Enable/disable LOAD DATA LOCAL INFILE.
-b, --no-beep Turn off beep on error.
-h, --host=name Connect to host.
...
-p, --password[=name]
Password to use when connecting to server. If password is
not given it's asked from the tty.
-W, --pipe Use named pipes to connect to server.
-P, --port=# Port number to use for connection or 0 for default to, in
order of preference, my.cnf, $MYSQL_TCP_PORT,
/etc/services, built-in default (3306).
...
-q, --quick Don't cache result, print it row by row. This may slow
down the server if the output is suspended. Doesn't use
history file.
-u, --user=name User for login if not current user.
-U, --safe-updates Only allow UPDATE and DELETE that uses keys.
-U, --i-am-a-dummy Synonym for option --safe-updates, -U.
-v, --verbose Write more. (-v -v -v gives the table output format).
-V, --version Output version information and exit.
-w, --wait Wait and retry if connection is down.
--connect-timeout=# Number of seconds before connection timeout.
--max-allowed-packet=#
The maximum packet length to send to or receive from
server.
mysql练习题
习题
学生表: Students
| Column Name | Type |
|---|---|
| student_id | int |
| student_name | varchar |
主键为 student_id（学生ID），该表内的每一行都记录有学校一名学生的信息。
create table if not exists Students (student_id int , student_name varchar(20));
科目表: Subjects
| Column Name | Type |
|---|---|
| subject_name | varchar |
主键为 subject_name（科目名称），每一行记录学校的一门科目名称。
Create table If Not Exists Subjects (subject_name varchar(20));
考试表: Examinations
| Column Name | Type |
|---|---|
| student_id | int |
| subject_name | varchar |
Create table If Not Exists Examinations (student_id int, subject_name varchar(20));
这张表压根没有主键，可能会有重复行。
学生表里的一个学生修读科目表里的每一门科目，而这张考试表的每一行记录就表示学生表里的某个学生参加了一次科目表里某门科目的测试。
要求写一段 SQL 语句，查询出==每个学生参加每一门科目测试的次数==，结果按 student_id 和 subject_name 排序。
查询结构格式如下所示：
Students table:
| student_id | student_name |
|---|---|
| 1 | Alice |
| 2 | Bob |
| 13 | John |
| 6 | Alex |
Truncate table Students;
insert into Students (student_id, student_name) values ('1', 'Alice');
insert into Students (student_id, student_name) values ('2', 'Bob');
insert into Students (student_id, student_name) values ('13', 'John');
insert into Students (student_id, student_name) values ('6', 'Alex');
Subjects table:
| subject_name |
|---|
| Math |
| Physics |
| Programming |
Truncate table Subjects;
insert into Subjects (subject_name) values ('Math');
insert into Subjects (subject_name) values ('Physics');
insert into Subjects (subject_name) values ('Programming');
Examinations table:
| student_id | subject_name |
|---|---|
| 1 | Math |
| 1 | Physics |
| 1 | Programming |
| 2 | Programming |
| 1 | Physics |
| 1 | Math |
| 13 | Math |
| 13 | Programming |
| 13 | Physics |
| 2 | Math |
| 1 | Math |
Truncate table Examinations;
insert into Examinations (student_id, subject_name) values ('1', 'Math');
insert into Examinations (student_id, subject_name) values ('1', 'Physics');
insert into Examinations (student_id, subject_name) values ('1', 'Programming');
insert into Examinations (student_id, subject_name) values ('2', 'Programming');
insert into Examinations (student_id, subject_name) values ('1', 'Physics');
insert into Examinations (student_id, subject_name) values ('1', 'Math');
insert into Examinations (student_id, subject_name) values ('13', 'Math');
insert into Examinations (student_id, subject_name) values ('13', 'Programming');
insert into Examinations (student_id, subject_name) values ('13', 'Physics');
insert into Examinations (student_id, subject_name) values ('2', 'Math');
insert into Examinations (student_id, subject_name) values ('1', 'Math');
Result table:
| student_id | student_name | subject_name | attended_exams |
|---|---|---|---|
| 1 | Alice | Math | 3 |
| 1 | Alice | Physics | 2 |
| 1 | Alice | Programming | 1 |
| 2 | Bob | Math | 1 |
| 2 | Bob | Physics | 0 |
| 2 | Bob | Programming | 1 |
| 6 | Alex | Math | 0 |
| 6 | Alex | Physics | 0 |
| 6 | Alex | Programming | 0 |
| 13 | John | Math | 1 |
| 13 | John | Physics | 1 |
| 13 | John | Programming | 1 |
结果表需包含所有学生和所有科目（即便测试次数为0）：
Alice 参加了 3 次数学测试, 2 次物理测试，以及 1 次编程测试；
Bob 参加了 1 次数学测试, 1 次编程测试，没有参加物理测试；
Alex 啥测试都没参加；
John 参加了数学、物理、编程测试各 1 次。
答案
三种方法join
cross join
join
inner join
select * from a,b
后面它可以不加 on 条件，不会报错
最终形成一个笛卡尔积
image-20200719152610320
left join
right join
会报错
# 1、令学生表挂接每个课程;
select *
from students s1
cross join subjects s2;
image-20200715232740271
# 2、再上基础上，连接考试表;
select *
from students s1
cross join subjects s2
left join examinations s3
on s1.student_id = s3.student_id and s2.subject_name = s3.subject_name;
image-20200715232809847
# 3、进行聚合分类;
select *
from students s1
cross join subjects s2
left join examinations s3
on s1.student_id = s3.student_id and s2.subject_name = s3.subject_name
group by s1.student_id,s2.subject_name;
# 体会 on 连接条件，及分组的逻辑
image-20200715232902817
# 4、分组计数，修正字段
select s1.student_id, s1.student_name, s2.subject_name, count(s3.subject_name) as attended_exams
from students s1
cross join subjects s2
left join examinations s3
on s1.student_id = s3.student_id and s2.subject_name = s3.subject_name
group by s1.student_id,s2.subject_name;
image-20200715232940275