<h2><center>数据库管理</center></h2>

------

## 一：数据类型

​	在MySQL数据库管理系统中，可以通过存储引擎来决定表的类型。同时，MySQL数据库管理系统也提供了数据类型决定表存储数据的类型。

### 1. 数值类型

**整数类型**

整数类型：TINYINT  SMALLINT   MEDIUMINT   INT   BIGINT

作用：用于存储用户的年龄、游戏的Level、经验值等

![](http://182.92.143.66:40072/directlink/img/mysql/images-202503110011.png)

**浮点数类型**

浮点数类型：FLOAT    DOUBLE

作用：用于存储用户的身高、体重、薪水等

![](http://182.92.143.66:40072/directlink/img/mysql/images-202503110012.png)

```SQL
float(5,3)       # 5宽度   3精度
注意：
	宽度不算小数点
	宽度-精度=整数

案例：
mysql> create database test;
Query OK, 1 row affected (0.00 sec)

mysql> use test
Database changed
mysql> create table t1(id float(6,2));
Query OK, 0 rows affected (0.00 sec)

mysql> insert into t1 values('2.22');
Query OK, 1 row affected (0.02 sec)
```

**定点数类型**

定点数类型：DEC

作用：定点数在MySQL内部以字符串形式存储，比浮点数更精确，适合用来表示货币等精度高的数据

**位类型**

位类型：BIT

作用：BIT（M）可以用来存放多位二进制数，M范围从1~64，如果不写默认为1位

### 2. 字符串类型

- CHAR系列：CHAR   VARCHAR
- TEXT系列：TINYTEXT   TEXT   MEDIUMTEXT   LONGTEXT
- BLOB系列：TINYBLOB   BLOB   MEDIUMBLOB   LONGBLOB
- BINARY系列：BINARY   VARBINARY

**枚举类型**

​	枚举类型：枚举列可以把一些不重复的字符串存储成一个预定义的集合

```SQL
mysql> create table enum_table(e ENUM('fish','apple','dog'));
Query OK, 0 rows affected (0.01 sec)

mysql> insert into enum_table(e) values('fish');
Query OK, 1 row affected (0.01 sec)

mysql> select * from enum_table;
+------+
| e    |
+------+
| fish |
+------+
1 row in set (0.00 sec)

mysql> insert into enum_table(e) values('nihao');
ERROR 1265 (01000): Data truncated for column 'e' at row 1
```

**时间和日期类型**

时间和日期类型：DATE、TIME、DATETIME、TIMESTAMP、YEAR

作用：用于存储用户的注册时间，文章的发布时间，文章的更新时间，员工的入职时间等

![](http://182.92.143.66:40072/directlink/img/mysql/images-202503110013.png)

```SQL
mysql> create table t2(
    -> id1 timestamp NOT NULL default CURRENT_TIMESTAMP,
    -> id2 datetime default NULL
    -> );
Query OK, 0 rows affected (0.00 sec)

mysql> insert into t2(id1) values('20250311104200');
Query OK, 1 row affected (0.00 sec)

mysql> select * from t2;
+---------------------+------+
| id1                 | id2  |
+---------------------+------+
| 2025-03-11 10:42:00 | NULL |
+---------------------+------+
1 row in set (0.00 sec)

	timestamp 类型的列还有个特性：默认情况下，在 insert, update 数据时，timestamp 列会自动以当前时间（CURRENT_TIMESTAMP）填充/更新。“自动”的意思就是，你不去管它，MySQL 会替你去处理。
扩展：
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2025-03-11 10:42:44 |
+---------------------+
1 row in set (0.00 sec)
```

## 二：表操作

**使用编辑器编辑指令**

```SQL
mysql> edit 
mysql> \e
```

**在mysql客户端内执行系统命令**

```SQL
mysql> system  ls
mysql> \!  ls
```


### 1. 案例

表：school.student1

```
字段		    字段		        字段
id             name	        sex		       age
1              tom		    male	        23		 记录
2              jack		    male	        21		 记录
3              alice	    female	        19		 记录
```

语法：

```SQL
create table 表名(自定义)(
		字段名1  类型[(宽度) 约束条件],
		字段名2  类型[(宽度) 约束条件],
		字段名3  类型[(宽度) 约束条件]
)[存储引擎 字符集];

1.在同一张表中，字段名是不能相同
2.宽度和约束条件可选
3.字段名和类型是必须的
```

创建库表：

```SQL
mysql> create database school;
Query OK, 1 row affected (0.00 sec)

mysql> use school
Database changed
mysql> create table student1(
    -> id int,
    -> name varchar(50),
    -> sex enum('m','f'),
    -> age int
    -> );
Query OK, 0 rows affected (0.01 sec)
```

查看库：

```SQL
mysql> show tables;
+------------------+
| Tables_in_school |
+------------------+
| student1         |
+------------------+
1 row in set (0.00 sec)
```

插入语法：

```SQL
insert into 表名(字段1,字段2...)  values(字段值列表...);
```

插入数据：

```SQL
mysql> insert into student1(id,name,sex,age) values(1,'wxin','m',26);
Query OK, 1 row affected (0.01 sec)
```

查看表结构：

```SQL
mysql> desc student1;
+-------+---------------+------+-----+---------+-------+
| Field | Type          | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| id    | int(11)       | YES  |     | NULL    |       |
| name  | varchar(50)   | YES  |     | NULL    |       |
| sex   | enum('m','f') | YES  |     | NULL    |       |
| age   | int(11)       | YES  |     | NULL    |       |
+-------+---------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

 
mysql> show create table student1;       
mysql> show table status like 'student1' \G
```

查询数据：

```SQL
mysql> select id,name,sex,age from student1;  //查询表中所有字段的值
+------+------+------+------+
| id   | name | sex  | age  |
+------+------+------+------+
|    1 | wxin | m    |   26 |
+------+------+------+------+
1 row in set (0.00 sec)

mysql> select * from student1;   //查询表中所有字段的值
+------+------+------+------+
| id   | name | sex  | age  |
+------+------+------+------+
|    1 | wxin | m    |   26 |
+------+------+------+------+
1 row in set (0.00 sec)

mysql> select name,age from student1;   //查询表中指定字段的值
+------+------+
| name | age  |
+------+------+
| wxin |   26 |
+------+------+
1 row in set (0.00 sec)
```

扩展插入：

```SQL
mysql> insert into student1 values(2,'wyu','f',25),(3,'wyi','m',24),(4,'wbo','m',23);   //顺序插入
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> insert into student1(name,age) values('zhangsan',22),('lisi',21);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0
```

### 2. 案例

表：school.student2

```
               字段名             数据类型
编号             id                   int
姓名			  name			      varchar(50)
出生年份	     born_year	          year
生日			  birthday		      date
上课时间	     class_time	         time
注册时间	     reg_time		     datetime
```

创建表：

```SQL
mysql> create table student2(
    -> id int,
    -> name varchar(50),
    -> born_year year,
    -> birthday date,
    -> class_time time,
    -> reg_time datetime
    -> );
Query OK, 0 rows affected (0.00 sec)
```

插入数据：

```SQL
mysql> insert into student2 values(1,'tom',now(),now(),now(),now());
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql> insert into student2 values(2,'jack',1982,19821120,123000,20140415162545);
Query OK, 1 row affected (0.00 sec)
```

表：school.student3

```
id     id              int
姓名	name	    varchar(50)
性别	sex		    enum('male','female')
爱好	hobby	    set('music','book','game','disc')
```

创建表：

```SQL
mysql> create table student3( 
    -> id int, 
    -> name varchar(50), 
    -> sex enum('m','f'), 
    -> hobby set('music','book','game','disc')
	-> );
Query OK, 0 rows affected (0.00 sec)
```

查看表结构：

```SQL
mysql> desc student3;
+-------+-----------------------------------+------+-----+---------+-------+
| Field | Type                              | Null | Key | Default | Extra |
+-------+-----------------------------------+------+-----+---------+-------+
| id    | int(11)                           | YES  |     | NULL    |       |
| name  | varchar(50)                       | YES  |     | NULL    |       |
| sex   | enum('m','f')                     | YES  |     | NULL    |       |
| hobby | set('music','book','game','disc') | YES  |     | NULL    |       |
+-------+-----------------------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> show create table student3\G
*************************** 1. row ***************************
       Table: student3
Create Table: CREATE TABLE `student3` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(50) DEFAULT NULL,
  `sex` enum('m','f') DEFAULT NULL,
  `hobby` set('music','book','game','disc') DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
```

插入数据：

```SQL
mysql> insert into student3 values(1,'tom','m','book,game');
Query OK, 1 row affected (0.00 sec)
mysql> insert into student3 values(2,'jack','m','film');
ERROR 1265 (01000): Data truncated for column 'hobby' at row 1
```

注意：

DESCRIBE查看表结构

```SQL
DESCRIBE 表名;
DESC 表名;
```

查看表详细结构：

```SQL
SHOW CREATE TABLE 表名;
```


## 三：表完整性约束

### 1. 作用

​	用于保证数据的完整性和一致性

### 2. 约束条件

- RPIMARY KEY（PK）：标识该字段为该表的主键，可以唯一的标识记录，不可以为空 UNIQUE+NOT NULL
- FOREIGN KEY（FK）：标识该字段为该表的外键，实现表与表（父表主键/子表1外键/子表2外键）之间的关联
- NOT NULL：标识该字段不能为空
- UNIQUE KEY（UK）：标识该字段的值是唯一的，可以为空，一个表中可以有多个UNIQUE KEY
- AUTO_INCREMENT：标识该字段的值自动增长（整数类型，而且为主键）
- DEFAULT：该字段设置默认值

注意：

- 是否允许为空，默认NULL，可设置NOT NULL，字段不允许为空，必须赋值
- 字段是否有默认值，缺省的默认值是NULL，如果插入记录时不给字段赋值，此字段使用默认值

```SQL
mysql> sex enum('male','female') not null default 'male'
mysql> age int unsigned NOT NULL default 20
必须为正值（无符号） 不允许为空  默认是20
```

​	是否是key		主键 primary key		外键 forengn key

### 3. NOT NULL

表：school.student4

创建表：

```SQL
mysql> create table student4(
    -> id int not null,
    -> name varchar(50) not null,
    -> sex enum('m','f') default 'm' not null,
    -> age int unsigned default 18 not null,
    -> hobby set('music','disc','dance','book') default 'book,dance'
    -> );
Query OK, 0 rows affected (0.01 sec)
```

插入数据：（注意观察查询到的数据）

```SQL
mysql> insert into student4(id,name) values(2,'robin');
Query OK, 1 row affected (0.00 sec)

mysql> select * from student4;
+----+-------+-----+-----+------------+
| id | name  | sex | age | hobby      |
+----+-------+-----+-----+------------+
|  2 | robin | m   |  18 | dance,book |
+----+-------+-----+-----+------------+
1 row in set (0.00 sec)
```

注意报错的原因：

```SQL
mysql> insert into student4 values(3,null,'m',40,'book');
ERROR 1048 (23000): Column 'name' cannot be null
```

### 4. 唯一约束

作用：

​	MySQL索引的建立对于MySQL的高效运行是很重要的，索引可以大大提高MySQL的检索速度

表：company.department

创建表：

```SQL
mysql> create database company;
Query OK, 1 row affected (0.00 sec)

mysql> create table company.department1(
    -> dept_id int,
    -> dept_name varchar(30) UNIQUE,
    -> comment varchar(50)
    -> );
Query OK, 0 rows affected (0.00 sec)
```

查看表结构：

```SQL
mysql> desc company.department1;
+-----------+-------------+------+-----+---------+-------+
| Field     | Type        | Null | Key | Default | Extra |
+-----------+-------------+------+-----+---------+-------+
| dept_id   | int(11)     | YES  |     | NULL    |       |
| dept_name | varchar(30) | YES  | UNI | NULL    |       |
| comment   | varchar(50) | YES  |     | NULL    |       |
+-----------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
```

数据插入：（注意查看插入数据时的提示）

```SQL
mysql> insert into company.department1 values(1,'zhangsan','hr');
Query OK, 1 row affected (0.00 sec)

mysql> insert into company.department1 values(1,'zhangsan','hr');
ERROR 1062 (23000): Duplicate entry 'zhangsan' for key 'dept_name'
```

### 5. 主键约束

注意：

​	primary key字段的值是不允许重复，且不允许不NULL（UNIQUE + NOT NULL）

表：school.student6

创建表：

```SQL
mysql> create table school.student6(
    -> id int primary key not null auto_increment,
    -> name varchar(50) not null,
    -> sex enum('m','f') not null default 'm',
    -> age int not null default 18
    -> );
Query OK, 0 rows affected (0.01 sec)
```

插入数据：

```SQL
mysql> insert into school.student6 values (1,'alice','f',22);
Query OK, 1 row affected (0.00 sec)

mysql> insert into school.student6(name,sex,age) values ('jack','m',19),('tom','m',23);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from school.student6;
+----+-------+-----+-----+
| id | name  | sex | age |
+----+-------+-----+-----+
|  1 | alice | f   |  22 |
|  2 | jack  | m   |  19 |
|  3 | tom   | m   |  23 |
+----+-------+-----+-----+
3 rows in set (0.00 sec)
```


## 四：修改表

语法格式：

- 修改表名

  1. alter table 表名 rename 新表名
  2. rename table 新表名 to 表名

- 增加字段

  1. alter table 表名

     ​	add 字段名 数据类型 [完整性约束条件...],

     ​	add 字段名 数据类型 [完整性约束条件...];

  2. alter table 表名 add 字段名 数据类型 [完整性约束条件...] after 字段名；

- 删除字段：alter table 表名 drop 字段名

- 修改字段

  1. alter table 表名 modify 字段名 数据类型 [完整性约束条件...];
  2. alter table 表名 change 旧字段名 新字段名 旧数据类型 [完整性约束条件...];
  3. alter table 表名 change 旧字段名 新字段名 新数据类型 [完整性数据约束...]：

### 1. 修改数据库引擎

```SQL
mysql> alter table student2 engine=myisam;
Query OK, 2 rows affected (0.01 sec)
Records: 2  Duplicates: 0  Warnings: 0
//engine=innodb|memory|....
```

### 2. 添加字段

```SQL
mysql> create table student5(id int);
Query OK, 0 rows affected (0.01 sec)

mysql> alter table student5 add name varchar(30) not null,add age int not null default 22;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table student5 add stu_num int not null after name;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table student5 add sex enum('m','f') default 'm' first;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0
```

### 3. 删除字段

```SQL
mysql> alter table student5 drop sex;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0
```

### 4. 修改字段类型

```SQL
mysql> desc student5;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id      | int(11)     | YES  |     | NULL    |       |
| name    | varchar(30) | NO   |     | NULL    |       |
| stu_num | int(11)     | NO   |     | NULL    |       |
| age     | int(11)     | NO   |     | 22      |       |
+---------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table student5 modify age tinyint not null;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc student5;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id      | int(11)     | YES  |     | NULL    |       |
| name    | varchar(30) | NO   |     | NULL    |       |
| stu_num | int(11)     | NO   |     | NULL    |       |
| age     | tinyint(4)  | NO   |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+
4 rows in set (0.01 sec)

mysql> alter table student5 modify id int not null primary key;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc student5;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id      | int(11)     | NO   | PRI | NULL    |       |
| name    | varchar(30) | NO   |     | NULL    |       |
| stu_num | int(11)     | NO   |     | NULL    |       |
| age     | tinyint(4)  | NO   |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
```

### 5. 增加约束

```SQL
mysql> desc student5;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id      | int(11)     | NO   | PRI | NULL    |       |
| name    | varchar(30) | NO   |     | NULL    |       |
| stu_num | int(11)     | NO   |     | NULL    |       |
| age     | tinyint(4)  | NO   |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table student5 modify id int not null auto_increment;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc student5;                                         +---------+-------------+------+-----+---------+----------------+
| Field   | Type        | Null | Key | Default | Extra          |
+---------+-------------+------+-----+---------+----------------+
| id      | int(11)     | NO   | PRI | NULL    | auto_increment |
| name    | varchar(30) | NO   |     | NULL    |                |
| stu_num | int(11)     | NO   |     | NULL    |                |
| age     | tinyint(4)  | NO   |     | NULL    |                |
+---------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
```

### 6. 增加主键

```SQL
mysql> desc student1;
+-------+---------------+------+-----+---------+-------+
| Field | Type          | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| id    | int(11)       | YES  |     | NULL    |       |
| name  | varchar(50)   | YES  |     | NULL    |       |
| sex   | enum('m','f') | YES  |     | NULL    |       |
| age   | int(11)       | YES  |     | NULL    |       |
+-------+---------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table student1 add primary key(id);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc student1;
+-------+---------------+------+-----+---------+-------+
| Field | Type          | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| id    | int(11)       | NO   | PRI | NULL    |       |
| name  | varchar(50)   | YES  |     | NULL    |       |
| sex   | enum('m','f') | YES  |     | NULL    |       |
| age   | int(11)       | YES  |     | NULL    |       |
+-------+---------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
```

### 7. 修改主键和自增

```SQL
mysql> alter table student1 modify id int auto_increment;
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> desc student1;
+-------+---------------+------+-----+---------+----------------+
| Field | Type          | Null | Key | Default | Extra          |
+-------+---------------+------+-----+---------+----------------+
| id    | int(11)       | NO   | PRI | NULL    | auto_increment |
| name  | varchar(50)   | YES  |     | NULL    |                |
| sex   | enum('m','f') | YES  |     | NULL    |                |
| age   | int(11)       | YES  |     | NULL    |                |
+-------+---------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
```

### 8. 删除主键

```SQL
mysql> desc student5;
+---------+-------------+------+-----+---------+----------------+
| Field   | Type        | Null | Key | Default | Extra          |
+---------+-------------+------+-----+---------+----------------+
| id      | int(11)     | NO   | PRI | NULL    | auto_increment |
| name    | varchar(30) | NO   |     | NULL    |                |
| stu_num | int(11)     | NO   |     | NULL    |                |
| age     | tinyint(4)  | NO   |     | NULL    |                |
+---------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

mysql> alter table student5 drop primary key;
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key
mysql> alter table student5 modify id int not null;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc student5;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id      | int(11)     | NO   | PRI | NULL    |       |
| name    | varchar(30) | NO   |     | NULL    |       |
| stu_num | int(11)     | NO   |     | NULL    |       |
| age     | tinyint(4)  | NO   |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
```

### 9. 复制表

复制表结构＋记录 （key不会复制: 主键、外键和索引）复制表结构/记录+表结构，不会将Key复制

```SQL
mysql> create table new_service select * from service;
```

只复制表结构

```SQL
mysql> create table new1_service select * from service where 1=2;   //条件为假，查不到任何记录
```

可以复制主键，只复制表结构

```SQL
mysql> create table t4 like employees;
```

### 10. 删除表

```SQL
mysql> DROP TABLE 表名;

mysql> show tables;
+------------------+
| Tables_in_school |
+------------------+
| student1         |
| student2         |
| student3         |
| student4         |
| student5         |
| student6         |
+------------------+
6 rows in set (0.00 sec)

mysql> drop table student5;
Query OK, 0 rows affected (0.00 sec)

mysql> show tables;
+------------------+
| Tables_in_school |
+------------------+
| student1         |
| student2         |
| student3         |
| student4         |
| student6         |
+------------------+
5 rows in set (0.00 sec)
```

### 11. 修改数据表中字段的值

语法：

update 表名 set 列名=值 where 条件

```SQL
mysql> update student set name='123' where id=1;
```

### 12. 删除某一行

语法：

delete from 表名 where id = 1

```SQL
mysql> delete from type where id=1;
```

## 五：库操作

### 1. 简介

​	系统自带库的含义及作用

```SQL
mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
7 rows in set (0.00 sec)
```

- information_schema：虚拟库，主要存储了系统中的一些数据库对象的信息，例如用户表信息、列信息、权限信息、字符信息等
- performance_schema：主要存储数据库服务器的性能参数
- mysql：授权库，主要存储系统用户的权限信息
- sys：主要存储数据库服务器的性能参数

注意：information_schema

​	SCHEMATA     存放的是系统中的库

```SQL
mysql> select * from information_schema.SCHEMATA;
+--------------+--------------------+----------------------------+------------------------+----------+
| CATALOG_NAME | SCHEMA_NAME        | DEFAULT_CHARACTER_SET_NAME | DEFAULT_COLLATION_NAME | SQL_PATH |
+--------------+--------------------+----------------------------+------------------------+----------+
| def          | information_schema | utf8                       | utf8_general_ci        | NULL     |
| def          | company            | utf8                       | utf8_general_ci        | NULL     |
| def          | mysql              | utf8                       | utf8_general_ci        | NULL     |
| def          | performance_schema | utf8                       | utf8_general_ci        | NULL     |
| def          | school             | utf8                       | utf8_general_ci        | NULL     |
| def          | sys                | utf8                       | utf8_general_ci        | NULL     |
| def          | test               | utf8                       | utf8_general_ci        | NULL     |
+--------------+--------------------+----------------------------+------------------------+----------+
7 rows in set (0.00 sec)

目录_名称
实际库_名称
默认_字符_设置_名称
默认_分类_名称
```

TABLES 存储表名

```SQL
mysql> select * from information_schema.TABLES\G
*************************** 1. row ***************************
  TABLE_CATALOG: def
   TABLE_SCHEMA: information_schema
     TABLE_NAME: CHARACTER_SETS
     TABLE_TYPE: SYSTEM VIEW
         ENGINE: MEMORY
        VERSION: 10
     ROW_FORMAT: Fixed
     TABLE_ROWS: NULL
 AVG_ROW_LENGTH: 384
    DATA_LENGTH: 0
MAX_DATA_LENGTH: 16434816
   INDEX_LENGTH: 0
      DATA_FREE: 0
 AUTO_INCREMENT: NULL
    CREATE_TIME: 2025-03-11 12:37:46
    UPDATE_TIME: NULL
     CHECK_TIME: NULL
TABLE_COLLATION: utf8_general_ci
       CHECKSUM: NULL
 CREATE_OPTIONS: max_rows=43690
  TABLE_COMMENT: 
```


### 2. 创建库

方案一：交互式操作

```SQL
mysql> create database wxin;
Query OK, 1 row affected (0.00 sec)
```

方案二：非交互式

```bash
[root@rpm mysql]# ./bin/mysql -uroot -p'Wfahui@3123' -e "create database wxin";
```

数据库命名规则：

- 区分大小写
- 唯一性
- 不能使用关键字如 create select
- 不能单独使用数字

**sql语句结尾**

​	每条sql语句都要以;结尾，但是如果列比较多，想看的清楚一点，可以以\G结尾

### 3. 查看数据库

```SQL
mysql> show databases;
mysql> show create database wxin;
mysql> select database(); 查看当前所在的库
```

### 4.切换数据库

```SQL
mysql> use wxin
```

### 5. 删除数据库

```SQL
mysql> DROP DATABASE 数据库名;
```