库操作

系统安装默认自带的库

　　information_schema：虚拟库，不占用磁盘空间，存储的是数据库启动后的一些参数，如用户表信息、列信息、权限信息、字符信息等
　　performance_schema： MySQL 5.5开始新增一个数据库：主要用于收集数据库服务器性能参数，记录处理查询请求时发生的各种事件、锁等现象
　　mysql：授权库，主要存储系统用户的权限信息

一、查看数据库

【1】查看所有数据库

　　语句：show databases;

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

【2】查看数据库的创建方式

　　语句：show create database db_name; db_name为库名

mysql> show create database hugo;
+----------+-----------------------------------------------------------------+
| Database | Create Database                                                 |
+----------+-----------------------------------------------------------------+
| hugo     | CREATE DATABASE `hugo` /*!40100 DEFAULT CHARACTER SET latin1 */ |
+----------+-----------------------------------------------------------------+
1 row in set (0.02 sec)

【3】查看当前使用的库

select database()

二、创建数据数据库

　创建数据库（在磁盘上对应的文件夹）

　　语句：create database [if not exists] db_name [default] character set [=] charset_name

mysql> CREATE DATABASE test;
Query OK, 1 row affected (0.00 sec)
 
mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| Cy_NewErp          |
| hugo               |
| mysql              |
| performance_schema |
| sys                |
| test               |
+--------------------+
7 rows in set (0.00 sec)

　当我们创建存在的库时，会出现报错

1 2	`mysql> SHOW` `DATABASE` `test;` `ERROR 1064 (42000): You have an error` `in` `your SQL syntax;` `check` `the manual that corresponds` `to` `your MySQL server version` `for` `the` `right` `syntax` `to` `use near` `'DATABASE test'` `at` `line 1`

mysql> CREATE DATABASE IF NOT  EXISTS test;
Query OK, 1 row affected, 1 warning (0.01 sec)
 
mysql> SHOW WARNINGS;    --查看警告
+-------+------+-----------------------------------------------+
| Level | Code | Message                                       |
+-------+------+-----------------------------------------------+
| Note  | 1007 | Can't create database 'test'; database exists |
+-------+------+-----------------------------------------------+
1 row in set (0.00 sec)

--我的电脑创建出来的库字符集默认是latin1   
mysql> SHOW CREATE DATABASE test;
+----------+-----------------------------------------------------------------+
| Database | Create Database                                                 |
+----------+-----------------------------------------------------------------+
| test     | CREATE DATABASE `test` /*!40100 DEFAULT CHARACTER SET latin1 */ |
+----------+-----------------------------------------------------------------+

mysql> CREATE DATABASE IF NOT EXISTS test1 CHARACTER SET UTF8;
Query OK, 1 row affected (0.00 sec)
 
mysql> SHOW CREATE DATABASE test1;
+----------+----------------------------------------------------------------+
| Database | Create Database                                                |
+----------+----------------------------------------------------------------+
| test1    | CREATE DATABASE `test1` /*!40100 DEFAULT CHARACTER SET utf8 */ |
+----------+----------------------------------------------------------------+
1 row in set (0.00 sec)

三、修改数据库　　

　语句：alter [database | schema] [db_name] [default] character set [=] charset_name　　

mysql> SHOW CREATE DATABASE test;
+----------+-----------------------------------------------------------------+
| Database | Create Database                                                 |
+----------+-----------------------------------------------------------------+
| test     | CREATE DATABASE `test` /*!40100 DEFAULT CHARACTER SET latin1 */ |
+----------+-----------------------------------------------------------------+
1 row in set (0.00 sec)
 
mysql> ALTER DATABASE test CHARACTER SET = UTF8;
Query OK, 1 row affected (0.00 sec)
 
mysql> SHOW CREATE DATABASE test;
+----------+---------------------------------------------------------------+
| Database | Create Database                                               |
+----------+---------------------------------------------------------------+
| test     | CREATE DATABASE `test` /*!40100 DEFAULT CHARACTER SET utf8 */ |
+----------+---------------------------------------------------------------+
1 row in set (0.00 sec)

四、删除数据库

　语句：drop database [if exists] db_name;

mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| Cy_NewErp          |
| hugo               |
| mysql              |
| performance_schema |
| sys                |
| test               |
| test1              |
+--------------------+
8 rows in set (0.01 sec)
 
--删除存在的库的时候会报错
mysql> DROP DATABASE test2;
ERROR 1008 (HY000): Can't drop database 'test2'; database doesn't exist
 
--显示错误
mysql> SHOW ERRORS;
+-------+------+-----------------------------------------------------+
| Level | Code | Message                                             |
+-------+------+-----------------------------------------------------+
| Error | 1008 | Can't drop database 'test2'; database doesn't exist |
+-------+------+-----------------------------------------------------+
1 row in set (0.00 sec)
 
--删除test1库，当不确定这个库是否存在时，可以加可选项
mysql> DROP DATABASE IF NOT EXISTS test1;
Query OK, 0 rows affected (0.00 sec)

五、使用数据库

【1】切换数据库：

　　　语句：use db_name;

　　　注意：进入到某个数据库后没办法再退回之前状态，但可以通过use进行切换　　

mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| Cy_NewErp          |
| hugo               |
| mysql              |
| performance_schema |
| sys                |
| test               |
+--------------------+
7 rows in set (0.00 sec)
 
mysql> USE test;
Database changed

【2】查看当前使用的数据库

　　　　语句：select database();

mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| test       |
+------------+
1 row in set (0.00 sec)

　　切换数据库，可以用use切换

mysql> USE sys;
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> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| sys        |
+------------+
1 row in set (0.00 sec)