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-<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
-
-### 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)
-```
-
-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)
-```
-
-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
-
-方案二：非交互式
-
-```bash
-[root@rpm mysql]# ./bin/mysql -uroot -p'Wfahui@3123' -e "create database wxin";
-```
-
-### 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;
-```
-