MySQL基础
1.关系型数据库介绍
1.1 数据结构模型
数据结构模型主要有：
- 层次模型
- 网状结构
- 关键模型
关系模型：
二维关系：row，columm
数据库管理系统：
- DBMS （DataBase Managment System）
关系：relational
关系型数据库管理系统：RDBMS
1.2 RDBMS专用名词
关系型数据库管理系统：
mysql：5.7版本之间都是开源 8.0后开始商业化
- MySQL：MySQL，MariaDB，Percona-Server，sqlserver，MSsql（micresoft sql）
- PostgreSQL：简称为pgsql
- Oracle
- MSSQL
非关系型数据库管理系统：
数据存放在内存当中 变量 key=value
- mongodb
- redis
- memcache
SQL：Structure Query Language，结构化查询语言
约束：constraint，向数据表提供的数据要遵守的限制
- 主键约束：一个或多个字段的组合，填入的数据必须能在本表中唯一标识本行。且必须提供数据，不能为空（NOT NULL）。
- 一个表只能存在一个
- 惟一键约束：一个或多个字段的组合，填入的数据必须能在本表中唯一标识本行。允许为空（NULL）
- 一个表可以存在多个
- 外键约束：一个表中可填充字段的数据取决于另一个表的主键中已有的数据
[En]
Foreign key constraint: data that can be filled in a field in one table depends on the data already in the primary key of another table*
- 检查性约束
索引：将表中的一个或多个字段中的数据复制一份另存，并且这些数据需要按特定次序排序存储
1.3 关系型数据库的常见组件
关系数据库的常见组件包括：
[En]
Common components of a relational database are:
- 数据库：database
- 表：table，由行（row）和列（column）组成
- 索引：index
- 视图：view
- 用户：user
- 权限：privilege
- 存储过程：procedure
- 存储函数：function
- 触发器：trigger
- 事件调度器：event scheduler
1.4 SQL语句
SQL语句有三种类型：
- DDL：Data Defination Language，数据定义语言
- DML：Data Manipulation Language，数据操纵语言
- DCL：Data Control Language，数据控制语言
SQL语句类型 对应操作 DDL CREATE:创建
DROP:删除
ALTER:修改 DML INSERT:向表中插入数据
DELETE:删除表中数据
UPDATE:更新表中数据
SELECT:查询表中数据 DCL GRANT:授权
REVOKE:移除授权
2.mysql安装与配置
2.1 mysql安装
mysql安装方式有三种：
- 源代码：编译安装
- 二进制打包：展开到特定路径，简单配置即可使用
[En]
package in binary format: expand to a specific path and use it after simple configuration*
- 由包管理器管理的包：
[En]
packages managed by the package manager:*
- rpm：有两种
- OS Vendor：操作系统发行商提供的
- 项目官方提供的
- deb
mysql安装步骤
[root@localhost ~]# wget http://dev.mysql.com/get/mysql57-community-release-el7-11.noarch.rpm
[root@localhost ~]# rpm -Uvh mysql57-community-release-el7-11.noarch.rpm
[root@localhost ~]# yum module disable mysql //禁用mysql
[root@localhost ~]# yum -y install mysql-community-server mysql-community-client mysql-community-common mysql-community-devel --nogpgcheck //下载一个服务端，客户端，命令行，软件包 //--nogpgche禁止进行gpgcheck
2.2 mysql配置
安装完后设置开机自启动
[root@localhost ~]# systemctl enable --now mysqld
[root@localhost ~]# systemctl status mysqld
● mysqld.service - MySQL Server
Loaded: loaded (/usr/lib/systemd/system/mysqld.service; enabled; vendor preset: disabled)
Active: active (running) since Mon 2022-07-25 14:34:30 CST; 8s ago
Docs: man:mysqld(8)
http://dev.mysql.com/doc/refman/en/using-systemd.html
Process: 70174 ExecStart=/usr/sbin/mysqld --daemonize --pid-file=/var/run/mysqld/mysqld.p>
Process: 70022 ExecStartPre=/usr/bin/mysqld_pre_systemd (code=exited, status=0/SUCCESS)
Main PID: 70176 (mysqld)
Tasks: 27 (limit: 23457)
Memory: 300.4M
CGroup: /system.slice/mysqld.service
└─70176 /usr/sbin/mysqld --daemonize --pid-file=/var/run/mysqld/mysqld.pid
Jul 25 14:34:28 localhost.localdomain systemd[1]: Starting MySQL Server...
Jul 25 14:34:30 localhost.localdomain systemd[1]: Started MySQL Server.
查看3306端口是否监听
[root@localhost ~]# ss -anlt | grep 3306
LISTEN 0 80 *:3306 *:*
在日志中找出密码
[root@localhost ~]# grep "password" /var/log/mysqld.log
2022-07-25T06:34:28.883599Z 1 [Note] A temporary password is generated for root@localhost: OFU+amdhV3Wr //临时密码
2022-07-25T06:35:33.642430Z 0 [Note] Shutting down plugin 'validate_password'
2022-07-25T06:35:34.759663Z 0 [Note] Shutting down plugin 'sha256_password'
2022-07-25T06:35:34.759665Z 0 [Note] Shutting down plugin 'mysql_native_password'
使用临时密码登录MySQL
[root@localhost ~]# mysql -uroot -pOFU+amdhV3Wr //-p后可以跟密码
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.7.38
Copyright (c) 2000, 2022, Oracle and/or its affiliates.
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.
mysql> //看到这样的标识表示登录进去了
修改mysql登录密码
mysql> set global validate_password_policy=0;
Query OK, 0 rows affected (0.00 sec)
mysql> set global validate_password_length=1;
Query OK, 0 rows affected (0.00 sec)
mysql> ALTER USER 'root'@'localhost' IDENTIFIED BY '123456';
Query OK, 0 rows affected (0.00 sec)
为避免mysql自动升级，需要卸载最开始安装的yum源
[root@localhost ~]# rpm -e mysql57-community-release
3.mysql的层序组成
- 客户端
- mysql：CLI交互式客户端程序
- mysql_secure_installation：安全初始化，强烈建议安装完以后执行此命令
- mysqldump：mysql备份工具
- mysqladmin
- 服务器端
- mysqld
3.1 mysql工具使用
//语法：mysql [OPTIONS] [database]
//常用的OPTIONS：
-uUSERNAME //指定用户名，默认为root
-hHOST //指定服务器主机，默认为localhost，推荐使用ip地址 //套接字
-pPASSWORD //指定用户的密码
-P# //指定数据库监听的端口，这里的#需用实际的端口号代替，如-P3307
-V //查看当前使用的mysql版本
-e //不登录mysql执行sql语句后退出，常用于脚本
-S //使用套接字连接，仅限于用于本地通讯
示例
//查看当前MySQL的版本
[root@localhost ~]# mysql -V
mysql Ver 14.14 Distrib 5.7.38, for Linux (x86_64) using EditLine wrapper
//指定服务器主机，默认为localhost，推荐使用ip地址 //套接字
[root@localhost ~]# mysql -uroot -p123456 -h127.0.0.1
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 6
Server version: 5.7.38 MySQL Community Server (GPL)
Copyright (c) 2000, 2022, Oracle and/or its affiliates.
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.
mysql>
//注意，不推荐直接在命令行里直接用-pPASSWORD的方式登录，而是使用-p选项，然后交互式输入密码
[root@localhost ~]# mysql -uroot -p -h127.0.0.1
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 7
Server version: 5.7.38 MySQL Community Server (GPL)
Copyright (c) 2000, 2022, Oracle and/or its affiliates.
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.
mysql>
//不登录mysql执行sql语句后退出，常用于脚本
[root@localhost ~]# mysql -uroot -p -h127.0.0.1 -e "show databases;"
Enter password:
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
+--------------------+
3.2 服务器监听的两种socket地址
socket 说明 ip socket 默认监听在tcp3306端口，支持远程通信 unix 监听在sock文件上（/tmp/mysql.sock，/var/lib/mysql/mysql.sock）仅支持本地通信
server地址只能是：localhost，127.0.0.1
4.mysql数据类型
MySQL中定义数据字段的类型对数据库的优化是非常重要的。MySQL支持多种类型，大致可以分为三类：数值、日期/时间和字符串(字符)类型。
4.1 数值类型
- MySQL支持所有标准SQL数值数据类型。这些类型包括严格数值数据类型(INTEGER、SMALLINT、DECIMAL和NUMERIC)，以及近似数值数据类型(FLOAT、REAL和DOUBLE PRECISION)。
- 关键字INT是INTEGER的同义词，关键字DEC是DECIMAL的同义词。BIT数据类型保存位字段值，并且支持MyISAM、MEMORY、InnoDB和BDB表。作为SQL标准的扩展，MySQL也支持整数类型TINYINT、MEDIUMINT和BIGINT。下面的表显示了需要的每个整数类型的存储和范围
类型 大小 范围（有符号） 范围（无符号） 用途 TINYINT 1 byte (-128,127) (0,255) 小整数值 SMALLINT 2 bytes (-32 768，32 767) (0，65 535) 大整数值 MEDIUMINT 3 bytes (-8 388 608，8 388 607) (0，4 294 967 295) 大整数值 INT或INTEGER 4 bytes (-2 147 483 648，2 147 483 647) (0，4 294 967 295) 大整数值 BIGINT 8 bytes (-9,223,372,036,854,775,808，9 223 372 036 854 775 807) (0，18 446 744 073 709 551 615) 极大整数值 FLOAT 4 bytes (-3.402 823 466 E+38，-1.175 494 351 E-38)，0，(1.175 494 351 E-38，3.402 823 466 351 E+38) 0，(1.175 494 351 E-38，3.402 823 466 E+38) 单精度
浮点数值 DOUBLE 8 bytes (-1.797 693 134 862 315 7 E+308，-2.225 073 858 507 201 4 E-308)，0，(2.225 073 858 507 201 4 E-308，1.797 693 134 862 315 7 E+308) 0，(2.225 073 858 507 201 4 E-308，1.797 693 134 862 315 7 E+308) 双精度
浮点数值 DECIMAL 对DECIMAL(M,D) ，如果M>D，为M+2否则为D+2 依赖于M和D的值 依赖于M和D的值 小点数
4.2 日期和时间类型
- 表示时间值的日期和时间类型为DATETIME、DATE、TIMESTAMP、TIME和YEAR。每个时间类型有一个有效值范围和一个”零”值，当指定不合法的MySQL不能表示的值时使用”零”值。TIMESTAMP类型有专有的自动更新特性，将在后面描述。
类型 大小（byte） 范围 格式 用途 DATE 3 1000-01-01/9999-12-31 YYYY-MM-DD 日期值 TIME 3 ‘-838:59:59’/’838:59:59’ HH:MM:SS 时间值或持续时间 YEAR 1 1901/2155 YYYY 年份值 DATETIME 8 1000-01-01 00:00:00/9999-12-31 23:59:59 YYYY-MM-DD HH:MM:SS 混合日期和时间值 TIMESTAMP 4 1970-01-01 00:00:00/2038
结束时间是第
2147483647
秒，北京时间
2038-1-19 11:14:07
，格林尼治时间 2038年1月19日 凌晨 03:14:07 YYYYMMDD HHMMSS 混合日期和时间值，时间戳
4.3 字符串类型
- 字符串类型指CHAR、VARCHAR、BINARY、VARBINARY、BLOB、TEXT、ENUM和SET。该节描述了这些类型如何工作以及如何在查询中使用这些类型。
类型 大小 用途 CHAR 0-255 bytes 定长字符串 VARCHAR 0-65535 bytes 变长字符串 TINYBLOB 0-255 bytes 不超过 255 个字符的二进制字符串 TINYTEXT 0-255 bytes 短文本字符串 BLOB 0-65 535 bytes 二进制形式的长文本数据 TEXT 0-65 535 bytes 长文本数据 MEDIUMBLOB 0-16 777 215 bytes 二进制形式的中等长度文本数据 MEDIUMTEXT 0-16 777 215 bytes 中等长度文本数据 LONGBLOB 0-4 294 967 295 bytes 二进制形式的极大文本数据 LONGTEXT 0-4 294 967 295 bytes 极大文本数据
注意：char(n) 和 varchar(n) 中括号中 n 代表字符的个数，并不代表字节个数，比如 CHAR(30) 就可以存储 30 个字符。
- CHAR 和 VARCHAR 类型类似，但它们保存和检索的方式不同。它们的最大长度和是否尾部空格被保留等方面也不同。在存储或检索过程中不进行大小写转换。
- BINARY 和 VARBINARY 类似于 CHAR 和 VARCHAR，不同的是它们包含二进制字符串而不要非二进制字符串。也就是说，它们包含字节字符串而不是字符字符串。这说明它们没有字符集，并且排序和比较基于列值字节的数值值。
- BLOB 是一个二进制大对象，可以容纳可变数量的数据。有 4 种 BLOB 类型：TINYBLOB、BLOB、MEDIUMBLOB 和 LONGBLOB。它们区别在于可容纳存储范围不同。
- 有 4 种 TEXT 类型：TINYTEXT、TEXT、MEDIUMTEXT 和 LONGTEXT。对应的这 4 种 BLOB 类型，可存储的最大长度不同，可根据实际情况选择。
5.mysql数据库操作
5.1 DDL操作
5.1.1 数据库操作
创建数据库
[root@localhost ~]# mysql -uroot -p //先进入MySQL数据库
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 10
Server version: 5.7.38 MySQL Community Server (GPL)
Copyright (c) 2000, 2022, Oracle and/or its affiliates.
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.
mysql> show databases; //查看数据库
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
+--------------------+
4 rows in set (0.00 sec)
mysql> create database zxr; //创建数据库
Query OK, 1 row affected (0.00 sec)
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
| zxr |
+--------------------+
5 rows in set (0.00 sec)
删除数据库
mysql> show databases; //查看数据库
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
| zxr |
+--------------------+
5 rows in set (0.00 sec)
mysql> drop database zxr; //删除数据库
Query OK, 0 rows affected (0.00 sec)
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
+--------------------+
4 rows in set (0.00 sec)
5.1.2 表操作
创建表之前需要进入一个数据库
mysql> create database zxr; //需要先创建一个数据库
Query OK, 1 row affected (0.00 sec)
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
| zxr |
+--------------------+
5 rows in set (0.00 sec)
mysql> use zxr; //进入数据库里
Database changed //进入成功
mysql> create table linux (id int not null ,name varchar(100) not null,age tinyint); //创建表
Query OK, 0 rows affected (0.01 sec)
mysql> show tables; //查看表
+---------------+
| Tables_in_zxr |
+---------------+
| linux |
+---------------+
1 row in set (0.00 sec)
删除表
mysql> show tables;
+---------------+
| Tables_in_zxr |
+---------------+
| linux |
+---------------+
1 row in set (0.00 sec)
mysql> drop table linux; //drop删除
Query OK, 0 rows affected (0.00 sec)
mysql> show tables;
Empty set (0.00 sec)
5.1.3 用户操作
mysql用户帐号由两部分组成，如’USERNAME’@’HOST’，表示此USERNAME只能从此HOST上远程登录
host值可以为
- IP地址，如127.0.0.1
- 通配符
- %：配置任意长度的任意字符，常用于设置允许从任何主机登录
- _:匹配任意单个字符
创建用户
数据库用户创建语法：CREATE USER 'username'@'host' [IDENTIFIED BY 'password'];
[root@localhost ~]# mysql -uroot -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 12
Server version: 5.7.38 MySQL Community Server (GPL)
Copyright (c) 2000, 2022, Oracle and/or its affiliates.
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.
mysql> create user 'zxr'@'127.0.0.1' identified by '123456'; //创建用户
Query OK, 0 rows affected (0.00 sec)
使用新创建的用户和密码登录mysql
[root@localhost ~]# mysql -uzxr -p123456 -h127.0.0.1
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 13
Server version: 5.7.38 MySQL Community Server (GPL)
Copyright (c) 2000, 2022, Oracle and/or its affiliates.
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.
mysql>
删除用户
[root@localhost ~]# mysql -uroot -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 14
Server version: 5.7.38 MySQL Community Server (GPL)
Copyright (c) 2000, 2022, Oracle and/or its affiliates.
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.
mysql> drop user 'zxr'@'127.0.0.1'; //删除用户
Query OK, 0 rows affected (0.00 sec)
mysql>
5.1.4 查看命令show
查看支持的所有字符
mysql> show character set;
+----------+---------------------------------+---------------------+--------+
| Charset | Description | Default collation | Maxlen |
+----------+---------------------------------+---------------------+--------+
| big5 | Big5 Traditional Chinese | big5_chinese_ci | 2 |
| dec8 | DEC West European | dec8_swedish_ci | 1 |
| cp850 | DOS West European | cp850_general_ci | 1 |
| hp8 | HP West European | hp8_english_ci | 1 |
| koi8r | KOI8-R Relcom Russian | koi8r_general_ci | 1 |
| latin1 | cp1252 West European | latin1_swedish_ci | 1 |
| latin2 | ISO 8859-2 Central European | latin2_general_ci | 1 |
| swe7 | 7bit Swedish | swe7_swedish_ci | 1 |
| ascii | US ASCII | ascii_general_ci | 1 |
| ujis | EUC-JP Japanese | ujis_japanese_ci | 3 |
查看当前数据库支持的所有存储引擎
mysql> show engines;
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| Engine | Support | Comment | Transactions | XA | Savepoints |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| InnoDB | DEFAULT | Supports transactions, row-level locking, and foreign keys | YES | YES | YES |
| MRG_MYISAM | YES | Collection of identical MyISAM tables | NO | NO | NO |
| MEMORY | YES | Hash based, stored in memory, useful for temporary tables | NO | NO | NO |
| BLACKHOLE | YES | /dev/null storage engine (anything you write to it disappears) | NO | NO | NO |
| MyISAM | YES | MyISAM storage engine | NO | NO | NO |
| CSV | YES | CSV storage engine | NO | NO | NO |
| ARCHIVE | YES | Archive storage engine | NO | NO | NO |
| PERFORMANCE_SCHEMA | YES | Performance Schema | NO | NO | NO |
| FEDERATED | NO | Federated MySQL storage engine | NULL | NULL | NULL |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
9 rows in set (0.00 sec)
查看数据库信息
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
| zxr |
+--------------------+
5 rows in set (0.01 sec)
不进入数据库而列出其包含的所有表
mysql> show tables from zxr;
+---------------+
| Tables_in_zxr |
+---------------+
| linux |
+---------------+
1 row in set (0.00 sec)
查看表结构
mysql> desc zxr.linux;
+-------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id | int(11) | NO | | NULL | |
| name | varchar(100) | NO | | NULL | |
| age | tinyint(4) | YES | | NULL | |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.03 sec)
查看表的创建命令
mysql> show create table zxr.linux;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------+
| linux | CREATE TABLE linux (
id int(11) NOT NULL,
name varchar(100) NOT NULL,
age tinyint(4) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
查看表的状态
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
| zxr |
+--------------------+
5 rows in set (0.00 sec)
mysql> use zxr; //进入数据库
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> show table status like 'linux'\G
*************************** 1. row ***************************
Name: linux
Engine: InnoDB
Version: 10
Row_format: Dynamic
Rows: 0
Avg_row_length: 0
Data_length: 16384
Max_data_length: 0
Index_length: 0
Data_free: 0
Auto_increment: NULL
Create_time: 2022-07-25 16:00:38
Update_time: NULL
Check_time: NULL
Collation: latin1_swedish_ci
Checksum: NULL
Create_options:
Comment:
1 row in set (0.00 sec)
5.1.5 如何获取帮助
语法：HELP keyword;
示例
mysql> help create database;
Name: 'CREATE DATABASE'
Description:
Syntax:
CREATE {DATABASE | SCHEMA} [IF NOT EXISTS] db_name
[create_option] ...
create_option: [DEFAULT] {
CHARACTER SET [=] charset_name
| COLLATE [=] collation_name
}
CREATE DATABASE creates a database with the given name. To use this
statement, you need the CREATE privilege for the database. CREATE
SCHEMA is a synonym for CREATE DATABASE.
URL: https://dev.mysql.com/doc/refman/5.7/en/create-database.html
5.2DML操作
DML操作包括增(INSERT)、删(DELETE)、改(UPDATE)、查(SELECT)，均属针对表的操作。
5.2.1 INSERT语句
mysql> use zxr;
Database changed
mysql> insert into linux (id,name,age)values(2,'jerry',23),(3,'lisi',25),(4,'zhangsan',30),(5,'zhangsan',20),(6,'wangwu',null); //一次可以插入多条记录
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql> select * from linux;
+----+----------+------+
| id | name | age |
+----+----------+------+
| 2 | jerry | 23 |
| 3 | lisi | 25 |
| 4 | zhangsan | 30 |
| 5 | zhangsan | 20 |
| 6 | wangwu | NULL |
+----+----------+------+
5 rows in set (0.00 sec)
5.2.2 SELECT语句
字段column表示法
表示符 代表什么 * 所有字段 as 字段别名，当表明很长时可以用别名来代替
示例
mysql> select id from linux;
+----+
| id |
+----+
| 2 |
| 3 |
| 4 |
| 5 |
| 6 |
+----+
5 rows in set (0.00 sec)
mysql> select * from linux;
+----+----------+------+
| id | name | age |
+----+----------+------+
| 2 | jerry | 23 |
| 3 | lisi | 25 |
| 4 | zhangsan | 30 |
| 5 | zhangsan | 20 |
| 6 | wangwu | NULL |
+----+----------+------+
5 rows in set (0.00 sec)
mysql> select id as 序号,name as 姓名 from linux; //临时修改
+--------+----------+
| 序号 | 姓名 |
+--------+----------+
| 2 | jerry |
| 3 | lisi |
| 4 | zhangsan |
| 5 | zhangsan |
| 6 | wangwu |
+--------+----------+
5 rows in set (0.00 sec)
mysql> select * from linux where age is null;
+----+--------+------+
| id | name | age |
+----+--------+------+
| 6 | wangwu | NULL |
+----+--------+------+
1 row in set (0.00 sec)
条件判断语句WHERE
操作类型 常用操作符 操作符 >，
Original: https://www.cnblogs.com/Their-own/p/16517765.html
Author: 事愿人为
Title: MySQL基础
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