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/ |
+--------------------------+--------------------------------+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)。
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 |
+-------+---------+