数据库系统原理与应用教程（021）—— MySQL 的数据库操作

数据库是一个专门存储数据对象的容器，包括：数据表、视图、触发器、存储过程等，其中数据表（table）是最基本的数据对象，所有的数据都存放在数据表中。在创建任何数据对象之前，都要先创建数据库，然后在数据库中创建所需要的各种对象。

一、创建数据库的命令

使用 CREATE DATABASE 语句创建数据库，语法格式如下：

CREATE DATABASE [IF NOT EXISTS] <数据库名>
[[DEFAULT] CHARACTER SET <字符集名>] [[DEFAULT] COLLATE <校对规则名>];

/* 说明： （1）[] 中的内容是可选的。 （2）<数据库名>：要创建数据库的名称。数据库名不能和一个已经存在的数据库同名，MySQL 不允许在同一个系统中创建两个相同名称的数据库。 （3）IF NOT EXISTS：只有该数据库不存在时才能执行创建操作，可以避免因为数据库已经存在而引起错误。 （4）[DEFAULT] CHARACTER SET：指定数据库的默认字符集。字符集是用来定义 MySQL 存储字符串的方式。该选项可以省略，如果省略就采用配置文件中指定的字符集。 （5）[DEFAULT] COLLATE：指定字符集的默认校对规则。校对规则用来定义比较字符串的方式，以解决排序和字符分组的问题。该选项可以省略，如果省略就采用配置文件中指定的校对规则。 */

1、查看 MySQL 默认的字符集和校对规则

-- 查看系统默认的字符集
mysql> show variables like 'char%';
+--------------------------+----------------------------------+
| Variable_name            | Value                            |
+--------------------------+----------------------------------+
| character_set_client     | utf8                             |
| character_set_connection | utf8                             |
| character_set_database   | utf8                             |
| character_set_filesystem | binary                           |
| character_set_results    | utf8                             |
| character_set_server     | utf8                             |
| character_set_system     | utf8                             |
| character_sets_dir       | /usr/local/mysql/share/charsets/ |
+--------------------------+----------------------------------+
8 rows in set (0.00 sec)

-- 查看系统默认的校对规则
mysql> show variables like 'COLL%'; 
+----------------------+-----------------+
| Variable_name        | Value           |
+----------------------+-----------------+
| collation_connection | utf8_general_ci |
| collation_database   | utf8_general_ci |
| collation_server     | utf8_general_ci |
+----------------------+-----------------+
3 rows in set (0.00 sec)

2、查看 MySQL 支持的字符集及校对规则

使用 show character set 命令可以查看 MySQL 支持的字符集及校对规则。

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 |
| sjis     | Shift-JIS Japanese              | sjis_japanese_ci    |      2 |
| hebrew   | ISO 8859-8 Hebrew               | hebrew_general_ci   |      1 |
| tis620   | TIS620 Thai                     | tis620_thai_ci      |      1 |
| euckr    | EUC-KR Korean                   | euckr_korean_ci     |      2 |
| koi8u    | KOI8-U Ukrainian                | koi8u_general_ci    |      1 |
| gb2312   | GB2312 Simplified Chinese       | gb2312_chinese_ci   |      2 |
| greek    | ISO 8859-7 Greek                | greek_general_ci    |      1 |
| cp1250   | Windows Central European        | cp1250_general_ci   |      1 |
| gbk      | GBK Simplified Chinese          | gbk_chinese_ci      |      2 |
| latin5   | ISO 8859-9 Turkish              | latin5_turkish_ci   |      1 |
| armscii8 | ARMSCII-8 Armenian              | armscii8_general_ci |      1 |
| utf8     | UTF-8 Unicode                   | utf8_general_ci     |      3 |
| ucs2     | UCS-2 Unicode                   | ucs2_general_ci     |      2 |
| cp866    | DOS Russian                     | cp866_general_ci    |      1 |
| keybcs2  | DOS Kamenicky Czech-Slovak      | keybcs2_general_ci  |      1 |
| macce    | Mac Central European            | macce_general_ci    |      1 |
| macroman | Mac West European               | macroman_general_ci |      1 |
| cp852    | DOS Central European            | cp852_general_ci    |      1 |
| latin7   | ISO 8859-13 Baltic              | latin7_general_ci   |      1 |
| utf8mb4  | UTF-8 Unicode                   | utf8mb4_general_ci  |      4 |
| cp1251   | Windows Cyrillic                | cp1251_general_ci   |      1 |
| utf16    | UTF-16 Unicode                  | utf16_general_ci    |      4 |
| utf16le  | UTF-16LE Unicode                | utf16le_general_ci  |      4 |
| cp1256   | Windows Arabic                  | cp1256_general_ci   |      1 |
| cp1257   | Windows Baltic                  | cp1257_general_ci   |      1 |
| utf32    | UTF-32 Unicode                  | utf32_general_ci    |      4 |
| binary   | Binary pseudo charset           | binary              |      1 |
| geostd8  | GEOSTD8 Georgian                | geostd8_general_ci  |      1 |
| cp932    | SJIS for Windows Japanese       | cp932_japanese_ci   |      2 |
| eucjpms  | UJIS for Windows Japanese       | eucjpms_japanese_ci |      3 |
| gb18030  | China National Standard GB18030 | gb18030_chinese_ci  |      4 |
+----------+---------------------------------+---------------------+--------+
41 rows in set (0.11 sec)

3、创建数据库 mydb（不指定字符集和校对规则）

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
4 rows in set (0.01 sec)

mysql> create database mydb;
Query OK, 1 row affected (0.04 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mydb               |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
5 rows in set (0.00 sec)

-- 使用 MySQL 默认的字符集，即：character_set_database 参数对应的字符集。
mysql> show create database mydb;
+----------+---------------------------------------------------------------+
| Database | Create Database                                               |
+----------+---------------------------------------------------------------+
| mydb     | CREATE DATABASE `mydb` /*!40100 DEFAULT CHARACTER SET utf8 */ |
+----------+---------------------------------------------------------------+
1 row in set (0.00 sec)

4、创建数据库 wgx（指定字符集和校对规则）

mysql> create database wgx character set latin1 collate latin1_general_ci;
Query OK, 1 row affected (2.18 sec)

mysql> show create database wgx;
+----------+--------------------------------------------------------------------------------+
| Database | Create Database                                                                          +----------+------------------------------------------------------------------------------+
| wgx      | CREATE DATABASE `wgx` /*!40100 DEFAULT CHARACTER SET latin1 COLLATE latin1_general_ci */ 
+----------+------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mydb               |
| mysql              |
| performance_schema |
| sys                |
| wgx                |
+--------------------+
6 rows in set (0.00 sec)

二、查看 MySQL 数据库

使用 show databases 命令可以查看已创建的数据库有哪些，命令如下：

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mydb               |
| mysql              |
| performance_schema |
| sys                |
| wgx                |
+--------------------+
6 rows in set (0.00 sec)

三、修改数据库

使用 alter database 命令可以修改数据库的属性，主要修改数据库的字符集和校对规则，该命令中各选项的含义和 create database 相同。语法如下：

ALTER DATABASE [数据库名]
[ DEFAULT ] CHARACTER SET <字符集名> |
[ DEFAULT ] COLLATE <校对规则名>

把 wgx 数据库的字符集修改为 gbk，校对规则修改为 gbk_chinese_ci。

mysql> alter database wgx default character set gbk default collate gbk_chinese_ci;
Query OK, 1 row affected (0.01 sec)

mysql> show create database wgx;
+----------+-------------------------------------------------------------+
| Database | Create Database                                             |
+----------+-------------------------------------------------------------+
| wgx      | CREATE DATABASE `wgx` /*!40100 DEFAULT CHARACTER SET gbk */ |
+----------+-------------------------------------------------------------+
1 row in set (0.00 sec)

四、选择数据库

由于表、视图等对象必须包含到某个数据库中，因此在创建对象和查看之前必须先选择对象所在的数据库。选择数据库的语法如下：

use 数据库名;

选择数据库 wgx，在数据库中创建表 t1：

mysql> use mydb;
Database changed
mysql> create table t1(id int);
Query OK, 0 rows affected (0.29 sec)

mysql> show tables;
+----------------+
| Tables_in_mydb |
+----------------+
| t1             |
+----------------+
1 row in set (0.00 sec)

五、删除数据库

删除数据库使用 drop database 命令，语法如下：

drop database 数据库名;

删除数据库 wgx：

mysql> drop database wgx;
Query OK, 0 rows affected (0.02 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mydb               |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
5 rows in set (0.00 sec)