数据库学习_mySQL

Posted 2021-06-29 Leslie X徐

mysql

mysql安装

可以在以下网站寻找ubuntu安装包
[https://packages.ubuntu.com/]

在Ubuntu系统下，利用apt-get install安装MySQL时使用以下指令

sudo apt-get install default-mysql-server

验证mysql服务器是否正常运行

sudo systemctl status mysql

保护MySQL
MySQL安装随附一个名为的脚本mysql_secure_installation，可让您轻松提高数据库服务器的安全性。

调用不带参数的脚本：

sudo mysql_secure_installation

命令 sudo mysql 进入程序，quit 退出。

授权操作

MariaDB [(none)]> grant all privileges on mysqldata.* to pi@localhost  identified by 'xwh1995' with grant option;

mysql  -p //使用密码登陆

类型

字段：
属性：
- int
- float/double
- char[8]/varchar[8] : 不常改变的使用varchar
- NULL

约束：给予某些字段赋予一些条件
- unique: 唯一
- default: 默认约束
- not null: 该字段不能为空
- primary key: 主键——指定这个表单里哪一个字段为主键，查询使用主键来查更快，比如学生的表单主键设置为学号id
- foreign key: 外键——连接其他表单中的某个字段

表达式

加减乘除
逻辑：or, and
like: 模糊查询
通配符：’%‘代替某某(多个)，’*‘所有，’_'代替某一个(必须有一个)

一般使用：where + 表达式
如：where

库操作

查询库：show databases;
创建库：create database database_name;
链接库：use database_name;
删除库：drop database_name;
查看当前使用的数据库: select database();

MariaDB [(none)]> CREATE DATABASE mysqldata;
Query OK, 1 row affected (0.001 sec)

MariaDB [(none)]> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| mysqldata          |
| performance_schema |
+--------------------+
4 rows in set (0.001 sec)

表操作

表操作	指令
查看数据库中所有表	show tables;
创建表	create table table_name(字段 类型 约束);
若创建字段需要使用中文	则在后面加上 charset=utf8;
查看表头信息	desc table_name; 或者 show columns from table_name;
删除表单	drop table_name;
重命名表单	alter table t1 rename t2;
增加字段	alter table 表名 add字段 类型 其他;

表操作	指令
查看表单内容	select * from table_name where 表达式;
查看表单根据排序	select * from MyClass order by id asc/desc;
查询前几行数据	select * from MyClass limit 0,2;
插入内容	insert into table_name(insert_element) values(insert_value)
更新表单内容	update table_name set update_element=update_value where 表达式;
删除表单内容	delete from table_name where 表达式;

MariaDB [(none)]> use mysqldata
MariaDB [(none)]> create table student(
    -> id int(4) not null primary key auto_increment,
    -> name char(20) not null,
    -> sex int(4) not null default '0',
    -> degree double(16,2)
    -> );

MariaDB [mysqldata]> desc student
    -> ;
+--------+--------------+------+-----+---------+----------------+
| Field  | Type         | Null | Key | Default | Extra          |
+--------+--------------+------+-----+---------+----------------+
| id     | int(4)       | NO   | PRI | NULL    | auto_increment |
| name   | char(20)     | NO   |     | NULL    |                |
| sex    | int(4)       | NO   |     | 0       |                |
| degree | double(16,2) | YES  |     | NULL    |                |
+--------+--------------+------+-----+---------+----------------+
4 rows in set (0.005 sec)

MariaDB [mysqldata]> select * from student;
+----+------+-----+--------+
| id | name | sex | degree |
+----+------+-----+--------+
|  1 | Tom  |   0 |  89.00 |
|  2 | John |   0 |  90.00 |
+----+------+-----+--------+
2 rows in set (0.001 sec)

MariaDB [mysqldata]> insert into tb_stu values
	-> (3,'Max',0,99),
    -> (4,'Jack',1,78),
    -> (5,'Babala',1,67);
Query OK, 3 rows affected (0.007 sec)
Records: 3  Duplicates: 0  Warnings: 0

MariaDB [mysqldata]> select * from tb_stu where sex=1;
+----+--------+-----+--------+
| id | name   | sex | degree |
+----+--------+-----+--------+
|  4 | Jack   |   1 |  78.00 |
|  5 | Babala |   1 |  67.00 |
+----+--------+-----+--------+
2 rows in set (0.001 sec)

MariaDB [mysqldata]> select * from tb_stu where sex=0;
+----+------+-----+--------+
| id | name | sex | degree |
+----+------+-----+--------+
|  1 | Tom  |   0 |  89.00 |
|  2 | John |   0 |  90.00 |
|  3 | Max  |   0 |  99.00 |
+----+------+-----+--------+
3 rows in set (0.001 sec)

MariaDB [mysqldata]> select * from tb_stu order by id desc;
+----+--------+-----+--------+
| id | name   | sex | degree |
+----+--------+-----+--------+
|  5 | Babala |   1 |  67.00 |
|  4 | Jack   |   1 |  78.00 |
|  3 | Max    |   0 |  99.00 |
|  2 | John   |   0 |  90.00 |
|  1 | Tom    |   0 |  89.00 |
+----+--------+-----+--------+
5 rows in set (0.000 sec)


MariaDB [mysqldata]> select * from tb_stu order by id asc;
+----+--------+-----+--------+
| id | name   | sex | degree |
+----+--------+-----+--------+
|  1 | Tom    |   0 |  89.00 |
|  2 | John   |   0 |  90.00 |
|  3 | Max    |   0 |  99.00 |
|  4 | Jack   |   1 |  78.00 |
|  5 | Babala |   1 |  67.00 |
+----+--------+-----+--------+
5 rows in set (0.001 sec)

MariaDB [mysqldata]> select * from tb_stu
    -> where degree>80 and degree<100;
+----+------+-----+--------+
| id | name | sex | degree |
+----+------+-----+--------+
|  1 | Tom  |   0 |  89.00 |
|  2 | John |   0 |  90.00 |
|  3 | Max  |   0 |  99.00 |
+----+------+-----+--------+
3 rows in set (0.001 sec)

MariaDB [mysqldata]> select * from tb_stu
    -> where degree<80;
+----+--------+-----+--------+
| id | name   | sex | degree |
+----+--------+-----+--------+
|  4 | Jack   |   1 |  78.00 |
|  5 | Babala |   1 |  67.00 |
+----+--------+-----+--------+
2 rows in set (0.001 sec)

MariaDB [mysqldata]>