数据库 --- mysql（03）-- mysql字符集、表操作（01）

MYSQL

1、mysql字符集

（1）简介：

MySQL字符集包括字符集（CHARACTER）和校对规则（COLLATION）两个概念：

• 字符集（CHARACTER）是一套编码
• 校对规则（COLLATION）是在字符集内用于比较字符的一套规则。

mysql字符集：
	latin1支持西欧字符、希腊字符等
	gbk支持中文简体字符
	big5支持中文繁体字符
	utf8几乎支持世界所有国家的字符。
	utf8mb4是真正意义上的utf-8

（2）命令：

<1> 查看字符集

mysql8.0 [(none)]>show variables like 'character%';
-- 查看所有像character..的变量

+--------------------------+--------------------------------+
| Variable_name            | Value                          |
+--------------------------+--------------------------------+
| character_set_client     | utf8mb4                        |
| character_set_connection | utf8mb4                        |
| character_set_database   | gbk                            |
| character_set_filesystem | binary                         |
| character_set_results    | utf8mb4                        |
| character_set_server     | gbk                            |
| character_set_system     | utf8mb3                        |
| character_sets_dir       | /usr/share/mysql-8.0/charsets/ |
+--------------------------+--------------------------------+
8 rows in set (0.00 sec)

<2> 修改默认字符集

mysql8.0 [(none)]>set character_set_server = utf8mb4;

mysql8.0 [(none)]>set character_set_database = utf8mb4;

mysql8.0 [(none)]>show variables like 'character%';
+--------------------------+--------------------------------+
| Variable_name            | Value                          |
+--------------------------+--------------------------------+
| character_set_client     | utf8mb4                        |
| character_set_connection | utf8mb4                        |
| character_set_database   | utf8mb4                        |
| character_set_filesystem | binary                         |
| character_set_results    | utf8mb4                        |
| character_set_server     | utf8mb4                        |
| character_set_system     | utf8mb3                        |
| character_sets_dir       | /usr/share/mysql-8.0/charsets/ |
+--------------------------+--------------------------------+

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附：utf8和utf8mb4的区别

MySQL在5.5.3之后增加了这个utf8mb4的编码，mb4就是most bytes 4的意思，专门用来兼容四字节的unicode。好在utf8mb4是utf8的超集，除了将编码改为utf8mb4外不需要做其他转换。

utf8 编码最大字符长度为 3 字节，如果遇到 4 字节的宽字符就会插入异常了。三个字节的 UTF-8 最大能编码的 Unicode 字符是 0xffff，也就是 Unicode 中的基本多文种平面(BMP)。

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2、数据库对象

命名规则：

• 必须以字母开头
• 可包括数字和特殊字符（_和$）
• 不要使用MySQL的保留字
• 同一Schema下的对象不能同名

3、表的基本操作

数据表的每行称为一条记录（record）；每一列称为一个字段（field）

（1）数据类型

MYSQL中，有三种主要的类型：文本、数值和日期/时间类型

常用数据类型：

（2）创建表

语法：

CREATE TABLE 表名(
    列名 列数据类型，
    列名 列数据类型
);

例：

mysql8.0 [student]>create table t1(id int,name char(20)); -- 单行创建

mysql8.0 [student]>create table t1(
    -> id int,name char(20)
    -> ,address char(50));
/* 多行创建 */

mysql8.0 [student]>show tables;
+-------------------+
| Tables_in_student |
+-------------------+
| t1                |
| t2                |
+-------------------+

（2）查看表

<1> 查看数据库中的所有表

语法：SHOW TABLES[FROM 数据库名][LIKE wild];

例：

mysql8.0 [student]>show tables from mysql like '%server%'; -- 查看mysql数据库中含server字段的tables
+----------------------------+
| Tables_in_mysql (%server%) |
+----------------------------+
| server_cost                |
| servers                    |
+----------------------------+

<2> 显示当前数据库中已有的数据表信息

[1] 语法：{DESCRIBE|DESC}  表名 [列名];

mysql8.0 [student]>describe student.t2;
+---------+----------+------+-----+---------+-------+
| Field   | Type     | Null | Key | Default | Extra |
+---------+----------+------+-----+---------+-------+
| id      | int      | YES  |     | NULL    |       |
| name    | char(20) | YES  |     | NULL    |       |
| address | char(50) | YES  |     | NULL    |       |
+---------+----------+------+-----+---------+-------+

[2] 语法：show columns from 表名称;

mysql8.0 [student]>show columns from student.t1;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int      | YES  |     | NULL    |       |
| name  | char(20) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+

（3）删除表

语法：DROP TABLE [IF EXISTS] 表名;

mysql8.0 [student]>drop table t1; -- 当前处在数据库中，不指定数据库默认删除本数据库内的表

mysql8.0 [student]>drop table student.t2;

mysql8.0 [student]>show tables;

（4）修改表结构 --- ALTER

mysql8.0 [student]>desc t1;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int      | YES  |     | NULL    |       |
| name  | char(20) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+

#修改列类型：ALTER TABLE 表名 MODIFY 列名 列类型;
mysql8.0 [student]>alter table t1 modify name varchar(30);

mysql8.0 [student]>desc t1;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | YES  |     | NULL    |       |
| name  | varchar(30) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

#增加列：ALTER TABLE 表名 ADD 列名 列类型;
mysql8.0 [student]>alter table t1 add birthdar date;

mysql8.0 [student]>desc t1;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | int         | YES  |     | NULL    |       |
| name     | varchar(30) | YES  |     | NULL    |       |
| birthdar | date        | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+

#删除列：ALTER TABLE 表名 DROP 列名;
mysql8.0 [student]>alter table t1 drop birthdar;

mysql8.0 [student]>desc t1;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | YES  |     | NULL    |       |
| name  | varchar(30) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

#修改列名：ALTER TABLE 表名 CHANGE 旧列名 新列名 列类型;
mysql8.0 [student]>alter table t1 change id score int;

mysql8.0 [student]>desc t1;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| score | int         | YES  |     | NULL    |       |
| name  | varchar(30) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

#修改表名
方式1：ALTER TABLE 表名 RENAME 新表名;
方式2：RENAME TABLE 表名 TO 新表名;
mysql8.0 [student]>alter table t1 rename sss;

mysql8.0 [student]>rename table t2 to aaa;

mysql8.0 [student]>show tables;
+-------------------+
| Tables_in_student |
+-------------------+
| aaa               |
| sss               |
+-------------------+

（5）复制表结构

<1> 语法：create table 新表名 like 源表

-- 将源表的表结构复制到新表
mysql8.0 [student]>create table qqq like aaa;

mysql8.0 [student]>desc qqq;
+---------+----------+------+-----+---------+-------+
| Field   | Type     | Null | Key | Default | Extra |
+---------+----------+------+-----+---------+-------+
| id      | int      | YES  |     | NULL    |       |
| name    | char(20) | YES  |     | NULL    |       |
| address | char(50) | YES  |     | NULL    |       |
+---------+----------+------+-----+---------+-------+

<2> 语法：create table 新表名 select * from 源表

-- 实现表结构的复制，甚至可以将源表的表记录拷贝到新表中
mysql8.0 [student]>select * from sss;
+-------+---------+
| score | name    |
+-------+---------+
|    34 | mmm     |
|    35 | wangwei |
+-------+---------+

mysql8.0 [student]>create table ttt select * from sss;

mysql8.0 [student]>desc ttt;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| score | int         | YES  |     | NULL    |       |
| name  | varchar(30) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

mysql8.0 [student]>select * from ttt;
+-------+---------+
| score | name    |
+-------+---------+
|    34 | mmm     |
|    35 | wangwei |
+-------+---------+

<3> 语法：insert into 表名 select * from 原表;

仅复制数据
mysql8.0 [student]>select * from qqq;

mysql8.0 [student]>insert into qqq select * from aaa;

mysql8.0 [student]>select * from qqq;
+------+-------+----------+
| id   | name  | address  |
+------+-------+----------+
|   11 | smith | nework   |
|   22 | wang  | shanghai |
+------+-------+----------+

---

例1：将student数据库中的aaa表的表结构复制到stuinfo数据库中的mm表中

mysql8.0 [student]>create database stuinfo;
mysql8.0 [stuinfo]>create table mm like student.aaa;

mysql8.0 [stuinfo]>desc mm;
+---------+----------+------+-----+---------+-------+
| Field   | Type     | Null | Key | Default | Extra |
+---------+----------+------+-----+---------+-------+
| id      | int      | YES  |     | NULL    |       |
| name    | char(20) | YES  |     | NULL    |       |
| address | char(50) | YES  |     | NULL    |       |
+---------+----------+------+-----+---------+-------+

例2：在stuinfo中创建msd表，将student的sss表结构和数据复制过去

mysql8.0 [(none)]>create table stuinfo.msd select * from student.sss;

mysql8.0 [(none)]>use stuinfo;
mysql8.0 [stuinfo]>desc msd;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| score | int         | YES  |     | NULL    |       |
| name  | varchar(30) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

mysql8.0 [stuinfo]>select * from msd;
+-------+---------+
| score | name    |
+-------+---------+
|    34 | mmm     |
|    35 | wangwei |
+-------+---------+

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