MySQL数据库的基础学习（二）数据表的操作指令

移动开发 2023-04-08 21:18:57 阅读次数: 0

事物的难度远远低于对事物的恐惧
数据库的内部存储结构主要分为数据库、数据表和数据，本文主要说明对数据表的指令操作。使用的数据库版本是8.0.23.

1.创建数据表

首先执行下面的命令，选择一个数据库进行操作。

use 数据库名称;

指令如下：

create table 数据表名称(字段1名称 字段1数据类型，字段2名称 字段2数据类型…………);

mysql> use student;
Database changed
mysql> create table student_info(
    -> _id int,
    -> age int,
    -> name varchar(50),
    -> score double
    -> );
Query OK, 0 rows affected (0.02 sec)

2.字段数据类型

int				整数
double			浮点数
varchar(长度)	字符
bit				位
data			时间
datatime		时间

3.查看所有的数据表

指令如下：

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

4.查看数据表的编码格式

指令如下：

show create table 数据表名称;

mysql> show create table student_info;
+--------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table        | Create Table                                                                                                                                                                                                          |
+--------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| student_info | CREATE TABLE `student_info` (
  `_id` int DEFAULT NULL,
  `age` int DEFAULT NULL,
  `name` varchar(50) DEFAULT NULL,
  `score` double DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+--------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)

5.修改数据表的编码格式

指令如下：

alter table 数据表名称 character set 编码格式;

mysql> create table temp(
    -> age int,
    -> name varchar(50)
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> show create table temp;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                         |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------+
| temp  | CREATE TABLE `temp` (
  `age` int DEFAULT NULL,
  `name` varchar(50) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> alter table temp character set gbk;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show create table temp;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                           |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| temp  | CREATE TABLE `temp` (
  `age` int DEFAULT NULL,
  `name` varchar(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=gbk |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

上面的代码首先创建了一个新的数据表temp，默认编码格式为utf8，然后将其修改为gbk的编码格式。

6.查看数据表结构

指令如下：

desc 数据表名称;

mysql> show tables;
+-------------------+
| Tables_in_student |
+-------------------+
| student_info      |
| temp              |
+-------------------+
2 rows in set (0.00 sec)

mysql> desc student_info;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| _id   | int         | YES  |     | NULL    |       |
| age   | int         | YES  |     | NULL    |       |
| name  | varchar(50) | YES  |     | NULL    |       |
| score | double      | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

7.修改数据表结构

  1. 增加列，即添加一个字段
    指令如下：
alter table 数据表名称 add 字段名称 字段类型;

mysql> desc student_info;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| _id   | int         | YES  |     | NULL    |       |
| age   | int         | YES  |     | NULL    |       |
| name  | varchar(50) | YES  |     | NULL    |       |
| score | double      | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table student_info add address varchar(100);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc student_info;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| _id     | int          | YES  |     | NULL    |       |
| age     | int          | YES  |     | NULL    |       |
| name    | varchar(50)  | YES  |     | NULL    |       |
| score   | double       | YES  |     | NULL    |       |
| address | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
  1. 修改长度/类型/约束
    这里先说明如何修改长度和类型，约束在下文说明，指令如下：
alter table 数据表名称 modify 字段名称 字段类型（长度）;

mysql> desc student_info;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| _id     | int          | YES  |     | NULL    |       |
| age     | int          | YES  |     | NULL    |       |
| name    | varchar(50)  | YES  |     | NULL    |       |
| score   | double       | YES  |     | NULL    |       |
| address | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

mysql> alter table student_info modify age double;
Query OK, 0 rows affected (0.17 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table student_info modify name varchar(80);
Query OK, 0 rows affected (0.11 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc student_info;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| _id     | int          | YES  |     | NULL    |       |
| age     | double       | YES  |     | NULL    |       |
| name    | varchar(80)  | YES  |     | NULL    |       |
| score   | double       | YES  |     | NULL    |       |
| address | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

上面的代码将age的类型修改成了double，name 的长度修改成了80.

  1. 修改列名，即修改字段名称
    指令如下：
alter table 数据表名称 change 旧的字段名称 新的字段名称 新的字段类型（长度）;

mysql> alter table student_info change age lenth int(20);
Query OK, 0 rows affected, 1 warning (0.19 sec)
Records: 0  Duplicates: 0  Warnings: 1

mysql> desc student_info;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| _id     | int          | YES  |     | NULL    |       |
| lenth   | int          | YES  |     | NULL    |       |
| name    | varchar(80)  | YES  |     | NULL    |       |
| score   | double       | YES  |     | NULL    |       |
| address | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
  1. 删除列，即删除字段名称
    指令如下：
alter table 数据表名称 drop 字段名称;

mysql> alter table student_info drop lenth;
Query OK, 0 rows affected (0.09 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc student_info;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| _id     | int          | YES  |     | NULL    |       |
| name    | varchar(80)  | YES  |     | NULL    |       |
| score   | double       | YES  |     | NULL    |       |
| address | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

删除了lenth字段。

8.修改数据表名称

指令如下：

rename table 旧的数据表名称 to 新的数据表名称;

mysql> show tables;
+-------------------+
| Tables_in_student |
+-------------------+
| student_info      |
| temp              |
+-------------------+
2 rows in set (0.00 sec)

mysql> rename table temp to flag;
Query OK, 0 rows affected (0.03 sec)

mysql> show tables;
+-------------------+
| Tables_in_student |
+-------------------+
| flag              |
| student_info      |
+-------------------+
2 rows in set (0.00 sec)

9.单表约束

  • 主键约束
    一个数据表中只有一个主键，被约束为主键得字段名称不可以为空，其实主键的约束就是为了保证一个列数据不重复。
    • 第一种—对已经存在的字段进行操作
      代码如下：
alter table 数据表名称 modify 字段名称 字段类型 primary key;

mysql> alter table student_info modify _id int primary key;
Query OK, 0 rows affected (0.08 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc student_info;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| _id     | int          | NO   | PRI | NULL    |       |
| name    | varchar(80)  | YES  |     | NULL    |       |
| score   | double       | YES  |     | NULL    |       |
| address | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

- 第二种---在创建字段的时候进行操作

指令如下：

alter table 数据表名称 add 字段名称 字段类型（长度） primary key;

mysql> alter table student_info add age int primary key;
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc student_info;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| _id     | int          | NO   |     | NULL    |       |
| name    | varchar(80)  | YES  |     | NULL    |       |
| score   | double       | YES  |     | NULL    |       |
| address | varchar(100) | YES  |     | NULL    |       |
| age     | int          | NO   | PRI | NULL    |       |
+---------+--------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

- 第三种---在创建数据表的时候进行操作

指令如下：

create table 数据表名称(字段1名称 字段1数据类型 primary key，字段2名称 字段2数据类型…………);

mysql> create table temp(
    -> age int,
    -> name varchar(50),
    -> lenth int primary key
    -> );
Query OK, 0 rows affected (0.05 sec)

mysql> desc temp;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| age   | int         | YES  |     | NULL    |       |
| name  | varchar(50) | YES  |     | NULL    |       |
| lenth | int         | NO   | PRI | NULL    |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
  • 唯一约束
    同样也是为了保证不重复，与主键不同的是可以控制多个字段不重复。
    关键字是 unique 添加操作与主键添加的操作类似。
  • 非空约束
    被约束的字段，必须要有数据不能为空。
    关键字是 not null 添加操作与主键添加的操作类似。
  • 删除约束
    • 删除主键约束
      指令如下：
alter table 数据表名称 drop primary key;

mysql> desc student_info;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| _id     | int          | NO   | PRI | NULL    |       |
| name    | varchar(80)  | YES  |     | NULL    |       |
| score   | double       | YES  |     | NULL    |       |
| address | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table student_info drop primary key;
Query OK, 0 rows affected (0.10 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc student_info;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| _id     | int          | NO   |     | NULL    |       |
| name    | varchar(80)  | YES  |     | NULL    |       |
| score   | double       | YES  |     | NULL    |       |
| address | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
  • 删除唯一约束
    指令如下：
alter table 数据表名称 drop index 字段名称;

mysql> desc student_info;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| _id     | int          | NO   |     | NULL    |       |
| name    | varchar(80)  | YES  | UNI | NULL    |       |
| score   | double       | YES  |     | NULL    |       |
| address | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table student_info drop index name;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc student_info;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| _id     | int          | NO   |     | NULL    |       |
| name    | varchar(80)  | YES  |     | NULL    |       |
| score   | double       | YES  |     | NULL    |       |
| address | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
  • 删除非空约束
    指令如下：
alter table 数据表名称 modify 字段名称 字段类型;

mysql> desc student_info;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| _id     | int          | NO   |     | NULL    |       |
| name    | varchar(80)  | YES  |     | NULL    |       |
| score   | double       | NO   |     | NULL    |       |
| address | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table student_info modify score double;
Query OK, 0 rows affected (0.09 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc student_info;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| _id     | int          | NO   |     | NULL    |       |
| name    | varchar(80)  | YES  |     | NULL    |       |
| score   | double       | YES  |     | NULL    |       |
| address | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

10.删除数据表

指令如下：

drop table 数据表名称;

mysql> show tables;
+-------------------+
| Tables_in_student |
+-------------------+
| flag              |
| student_info      |
| temp              |
+-------------------+
3 rows in set (0.00 sec)

mysql> drop table flag;
Query OK, 0 rows affected (0.03 sec)

mysql> show tables;
+-------------------+
| Tables_in_student |
+-------------------+
| student_info      |
| temp              |
+-------------------+
2 rows in set (0.00 sec)

结语：
本文主要是针对数据库的一些基础指令操作，后续将进行说明针对于数据等指令操作。

可以参考下方列出的我的其他MySQL文章：

MySQL数据库8.0.23 新手详细安装步骤

MySQL数据库的基础学习（一）数据库的操作指令

MySQL数据库的基础学习（三）数据的操作指令

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