MySQL基础操作

最后更新时间：2022年10月23日

数据库基本操作

  1. 查看当前所有存在的数据库：SHOW DATABASES;

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

  2. 创建数据库：CREATE DATABASE database_name;

    mysql> create database ronie;
Query OK, 1 row affected (0.00 sec)

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

  3. 使用数据库：USE database_name;

    mysql> use ronie;
Database changed
mysql>

  4. 删除数据库：DROP DATABASE database_name;

    mysql> drop database ronie;
Query OK, 0 rows affected (0.01 sec)

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

数据库表的基本操作

创建数据表

  1. 查看数据库表：show tables；

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

  2. 创建数据表：CREATE TABLE name；

    • 要创建的表的名称，不区分大小写，不能使用SQL语言中的关键字，如DROP、ALTER、INSERT等。
    • 数据表中每一列（字段）的名称和数据类型，如果创建多列，就要用逗号隔开。
    mysql> create table user_info
 -> (
 -> id int(10),
 -> name varchar(25),
 -> age int(3)
 -> );
Query OK, 0 rows affected, 2 warnings (0.01 sec)

    PS 查看警告

    mysql> show warnings;
+---------+------+------------------------------------------------------------------------------+
| Level   | Code | Message                                                                      |
+---------+------+------------------------------------------------------------------------------+
| Warning | 1681 | Integer display width is deprecated and will be removed in a future release. |
| Warning | 1681 | Integer display width is deprecated and will be removed in a future release. |
+---------+------+------------------------------------------------------------------------------+
 2 rows in set (0.00 sec)

mysql>

  3. 设置主键

    • 主键，又称主码，是表中一列或多列的组合。主键约束（Primary Key Constraint）要求主键列的数据唯一，并且不允许为空。主键能够唯一地标识表中的一条记录，可以结合外键来定义不同数据表之间的关系，并且可以加快数据库查询的速度。主键和记录之间的关系如同身份证和人之间的关系，它们之间是一一对应的。主键分为两种类型：单字段主键和多字段联合主键。

    • 单字段主键主键由一个字段组成，SQL语句格式分为以下两种情况。以下两个例子执行后的结果是一样的，都会在id字段上设置主键约束。

      • 在定义列的同时指定主键，语法规则如下：字段名 数据类型 PRIMARY KEY [默认值]，重新定义table user_info：

        mysql> create table user_info
 -> (
 -> id int(10) primary key,
 -> name varchar(25),
 -> age int(3)
 -> );
Query OK, 0 rows affected, 2 warnings (0.00 sec)

      • 在定义完所有列之后指定主键：[CONSTRAINT <约束名>] PRIMARY KEY [字段名]，重新定义table user_info：

        mysql> create table user_info
 -> (
 -> id int(10),
 -> name varchar(25),
 -> age int(3),
 -> primary key(id)
 -> );
Query OK, 0 rows affected, 2 warnings (0.01 sec)

    • 多字段联合主键：PRIMARY KEY [字段1, 字段2,. . ., 字段n]

      mysql> create table user_info
-> (
-> id int(10),
-> name varchar(25),
-> age int(3),
-> primary key(id, name)
-> );
Query OK, 0 rows affected, 2 warnings (0.01 sec)

  4. 使用外键约束
    外键：

    • 外键用来在两个表的数据之间建立连接，可以是一列或者多列。一个表可以有一个或多个外键。外键对应的是参照完整性，一个表的外键可以为空值，若不为空值，则每一个外键值必须等于另一个表中主键的某个值。
    • 外键：首先它是表中的一个字段，虽可以不是本表的主键，但要对应另外一个表的主键。外键的主要作用是保证数据引用的完整性，定义外键后，不允许删除在另一个表中具有关联关系的行。外键的作用是保持数据的一致性、完整性。
    • 主表（父表）：对于两个具有关联关系的表而言，相关联字段中主键所在的那个表即是主表。从表（子表）：对于两个具有关联关系的表而言，相关联字段中外键所在的那个表即是从表。

    创建外键的语法：[CONSTRAINT <外键名>] FOREIGN KEY 字段名1 [ ,字段名2,…] REFERENCES <主表名> 主键列1 [ ,主键列2,…]

    mysql> create table department
 -> (
 -> id int(10),
 -> department_name varchar(25),
 -> location varchar(20),
 -> primary key(id)
 -> );
Query OK, 0 rows affected, 1 warning (0.01 sec)

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

mysql> create table user_info
 -> (
 -> id int(10),
 -> name varchar(25),
 -> department_id int(10),
 -> age int(3),
 -> constraint fk_user_department foreign key(department_id) references department(id)
 -> );
Query OK, 0 rows affected, 3 warnings (0.02 sec)

mysql> show warnings;
+---------+------+------------------------------------------------------------------------------+
| Level   | Code | Message                                                                      |
+---------+------+------------------------------------------------------------------------------+
| Warning | 1681 | Integer display width is deprecated and will be removed in a future release. |
| Warning | 1681 | Integer display width is deprecated and will be removed in a future release. |
| Warning | 1681 | Integer display width is deprecated and will be removed in a future release. |
+---------+------+------------------------------------------------------------------------------+
 3 rows in set (0.00 sec)

mysql>

  5. 使用非空约束
    非空约束（Not Null Constraint）指字段的值不能为空。对于使用了非空约束的字段，如果用户在添加数据时没有指定值，数据库系统会报错。语法规则：字段名 数据类型 not null。

    mysql> create table user
 -> (
 -> id int(10) not null,
 -> name varchar(250) not null,
 -> password varchar(250) not null,
 -> age int(3)
 -> );
Query OK, 0 rows affected, 2 warnings (0.01 sec)

  6. 使用唯一性约束
    唯一性约束（Unique Constraint）要求该列唯一，允许为空，但只能出现一个空值。唯一约束可以确保一列或者几列不出现重复值。在定义完列之后直接指定唯一约束，语法规则如下：字段名 数据类型 UNIQUE。一个表中可以有多个字段声明为UNIQUE，但只能有一个PRIMARY KEY声明；声明为PRIMAY KEY的列不允许有空值，但是声明为UNIQUE的字段允许空值（NULL）的存在。

    mysql> create table user
 -> (
 -> id int(10) not null unique,
 -> name varchar(250) not null,
 -> password varchar(250) not null,
 -> age int(3)
 -> );
Query OK, 0 rows affected, 2 warnings (0.01 sec)

  7. 使用默认约束
    默认约束（Default Constraint）指定某列的默认值。如果插入一条新的记录时没有为这个字段赋值，那么系统会自动为这个字段赋值。

    mysql> drop table user;
Query OK, 0 rows affected (0.01 sec)

mysql> create table user
 -> (
 -> id int(10) not null unique,
 -> name varchar(250) not null,
 -> password varchar(250) default 123456,
 -> age int(3) 
 -> );
Query OK, 0 rows affected, 2 warnings (0.01 sec)

  8. 设置表的属性值自动增加
    在数据库应用中，我们可能希望在每次插入新记录时，系统自动生成字段的主键值。可以通过为表主键添加AUTO_INCREMENT关键字来实现。默认的，在MySQL中AUTO_INCREMENT的初始值是1，每新增一条记录，字段值自动加1。一个表只能有一个字段使用AUTO_INCREMENT约束，且该字段必须为主键的一部分。AUTO_INCREMENT约束的字段可以是任何整数类型（TINYINT、SMALLIN、INT、BIGINT等）。

    mysql> create table user
 -> (
 -> id int(10) primary key auto_increment,
 -> name varchar(250) not null,
 -> password varchar(250) default 123456,
 -> age int(3)
 -> );
Query OK, 0 rows affected, 2 warnings (0.02 sec)

    上述数据表创建后。表中的id字段的值在添加记录的时候会自动增加，在插入记录的时候，默认的自增字段id的值从1开始，每次添加一条新记录，该值自动加1。

    mysql> insert into user (name, age) values ('ronie', 18), ('kelly', 20), ('jack', 24);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from user;
+----+-------+----------+------+
| id | name  | password | age  |
+----+-------+----------+------+
|  1 | ronie | 123456   |   18 |
|  2 | kelly | 123456   |   20 |
|  3 | jack  | 123456   |   24 |
+----+-------+----------+------+
 3 rows in set (0.00 sec)

mysql>

查看数据表结构

  1. DESCRIBE/DESC语句可以查看表的字段信息，其中包括字段名、字段数据类型、是否为主键、是否有默认值等。语法规则如下：DESCRIBE 表名；DESC 表名;

    mysql> describe user;
+----------+--------------+------+-----+---------+----------------+
| Field    | Type         | Null | Key | Default | Extra          |
+----------+--------------+------+-----+---------+----------------+
| id       | int          | NO   | PRI | NULL    | auto_increment |
| name     | varchar(250) | NO   |     | NULL    |                |
| password | varchar(250) | YES  |     | 123456  |                |
| age      | int          | YES  |     | NULL    |                |
+----------+--------------+------+-----+---------+----------------+
 4 rows in set (0.00 sec)

mysql> desc user;
+----------+--------------+------+-----+---------+----------------+
| Field    | Type         | Null | Key | Default | Extra          |
+----------+--------------+------+-----+---------+----------------+
| id       | int          | NO   | PRI | NULL    | auto_increment |
| name     | varchar(250) | NO   |     | NULL    |                |
| password | varchar(250) | YES  |     | 123456  |                |
| age      | int          | YES  |     | NULL    |                |
+----------+--------------+------+-----+---------+----------------+
 4 rows in set (0.00 sec)

mysql>
    • NULL：表示该列是否可以存储NULL值。
    • Key：表示该列是否已编制索引。
    • PRI表示该列是表主键的一部分；
    • UNI表示该列是UNIQUE索引的一部分；
    • MUL表示在列中某个给定值允许出现多次。
    • Default：表示该列是否有默认值，有的话指定值是多少。
    • Extra：表示可以获取的与给定列有关的附加信息，例如AUTO_INCREMENT等。

  1. 查看表详细结构语句SHOW CREATE TABLE

    mysql> show create user;
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 '' at line 1
mysql> show create table user;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                             |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| user  | CREATE TABLE `user` (
  `id` int NOT NULL AUTO_INCREMENT,
  `name` varchar(250) NOT NULL,
  `password` varchar(250) DEFAULT '123456',
  `age` int DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
 1 row in set (0.00 sec)

mysql> 

    mysql> show create table user\G;
*************************** 1. row ***************************
   Table: user
Create Table: CREATE TABLE `user` (
 `id` int NOT NULL AUTO_INCREMENT,
 `name` varchar(250) NOT NULL,
 `password` varchar(250) DEFAULT '123456',
 `age` int DEFAULT NULL,
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)

ERROR: 
No query specified

mysql>

修改数据表

  1. 修改表名，MySQL是通过ALTER TABLE语句来实现表名的修改的，ALTER TABLE <旧表名> RENAME [TO] <新表名>;

    mysql> show tables;
+-----------------+
| Tables_in_ronie |
+-----------------+
| department      |
| user            |
| user_info       |
+-----------------+
 3 rows in set (0.00 sec)

mysql> alter table user rename user_login_info;
Query OK, 0 rows affected (0.01 sec)

mysql> show tables;
+-----------------+
| Tables_in_ronie |
+-----------------+
| department      |
| user_info       |
| user_login_info |
+-----------------+
 3 rows in set (0.00 sec)

mysql>

  2. 修改字段的数据类型
    修改字段的数据类型，就是把字段的数据类型转换成另一种数据类型。语法：ALTER TABLE <表名> MODIFY <字段名>　<数据类型>。其中，“表名”指要修改数据类型的字段所在表的名称，“字段名”指需要修改的字段，“数据类型”指修改后字段的新数据类型。

    mysql> show tables;
+-----------------+
| Tables_in_ronie |
+-----------------+
| department      |
| user_info       |
| user_login_info |
+-----------------+
 3 rows in set (0.00 sec)

mysql> desc user_info;
+---------------+-------------+------+-----+---------+-------+
| Field         | Type        | Null | Key | Default | Extra |
+---------------+-------------+------+-----+---------+-------+
| id            | int         | YES  |     | NULL    |       |
| name          | varchar(25) | YES  |     | NULL    |       |
| department_id | int         | YES  | MUL | NULL    |       |
| age           | int         | YES  |     | NULL    |       |
+---------------+-------------+------+-----+---------+-------+
 4 rows in set (0.00 sec)

mysql> alter table user_info modify name varchar(250);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc user_info;
+---------------+--------------+------+-----+---------+-------+
| Field         | Type         | Null | Key | Default | Extra |
+---------------+--------------+------+-----+---------+-------+
| id            | int          | YES  |     | NULL    |       |
| name          | varchar(250) | YES  |     | NULL    |       |
| department_id | int          | YES  | MUL | NULL    |       |
| age           | int          | YES  |     | NULL    |       |
+---------------+--------------+------+-----+---------+-------+
 4 rows in set (0.00 sec)

mysql>

  3. 修改字段名
    MySQL中修改表字段名的语法规则如下：ALTER TABLE <表名> CHANGE <旧字段名> <新字段名> <新数据类型>;。其中，“旧字段名”指修改前的字段名；“新字段名”指修改后的字段名；“新数据类型”指修改后的数据类型，如果不需要修改字段的数据类型，将新数据类型设置成与原来一样即可，但数据类型不能为空。

    mysql> show tables;
+-----------------+
| Tables_in_ronie |
+-----------------+
| department      |
| user_info       |
| user_login_info |
+-----------------+
 3 rows in set (0.00 sec)

mysql> desc user_info;
+---------------+--------------+------+-----+---------+-------+
| Field         | Type         | Null | Key | Default | Extra |
+---------------+--------------+------+-----+---------+-------+
| id            | int          | YES  |     | NULL    |       |
| name          | varchar(250) | YES  |     | NULL    |       |
| department_id | int          | YES  | MUL | NULL    |       |
| age           | int          | YES  |     | NULL    |       |
+---------------+--------------+------+-----+---------+-------+
 4 rows in set (0.00 sec)

mysql> alter table user_info change name username varchar(200);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc user_info;
+---------------+--------------+------+-----+---------+-------+
| Field         | Type         | Null | Key | Default | Extra |
+---------------+--------------+------+-----+---------+-------+
| id            | int          | YES  |     | NULL    |       |
| username      | varchar(200) | YES  |     | NULL    |       |
| department_id | int          | YES  | MUL | NULL    |       |
| age           | int          | YES  |     | NULL    |       |
+---------------+--------------+------+-----+---------+-------+
 4 rows in set (0.00 sec)

mysql>

  4. 添加字段
    语法：ALTER TABLE <表名> ADD <新字段名> <数据结构> [约束条件] [FIRST | AFTER 已存在字段名]。一个完整字段包括字段名、数据类型、完整性约束。新字段名为需要添加的字段的名称；FIRST为可选参数，其作用是将新添加的字段设置为表的第一个字段；AFTER为可选参数，其作用是将新添加的字段添加到指定的“已存在字段名”的后面。

    • 添加无完整性约束条件的字段

      mysql> show tables;
+-----------------+
| Tables_in_ronie |
+-----------------+
| department      |
| user_info       |
| user_login_info |
+-----------------+
 3 rows in set (0.00 sec)

mysql> desc department;
+-----------------+-------------+------+-----+---------+-------+
| Field           | Type        | Null | Key | Default | Extra |
+-----------------+-------------+------+-----+---------+-------+
| id              | int         | NO   | PRI | NULL    |       |
| department_name | varchar(25) | YES  |     | NULL    |       |
| location        | varchar(20) | YES  |     | NULL    |       |
+-----------------+-------------+------+-----+---------+-------+
 3 rows in set (0.00 sec)

mysql> alter table department add avg_salary float after department_name;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc department;
+-----------------+-------------+------+-----+---------+-------+
| Field           | Type        | Null | Key | Default | Extra |
+-----------------+-------------+------+-----+---------+-------+
| id              | int         | NO   | PRI | NULL    |       |
| department_name | varchar(25) | YES  |     | NULL    |       |
| avg_salary      | float       | YES  |     | NULL    |       |
| location        | varchar(20) | YES  |     | NULL    |       |
+-----------------+-------------+------+-----+---------+-------+
 4 rows in set (0.00 sec)

mysql>

    • 添加有完整性约束条件的字段

      mysql> desc department;
+-----------------+-------------+------+-----+---------+-------+
| Field           | Type        | Null | Key | Default | Extra |
+-----------------+-------------+------+-----+---------+-------+
| id              | int         | NO   | PRI | NULL    |       |
| department_name | varchar(25) | YES  |     | NULL    |       |
| avg_salary      | float       | YES  |     | NULL    |       |
| location        | varchar(20) | YES  |     | NULL    |       |
+-----------------+-------------+------+-----+---------+-------+
 4 rows in set (0.00 sec)

mysql> alter table department add num_of_people int(3) default 0 after avg_salary;
Query OK, 0 rows affected, 1 warning (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 1

mysql> desc department;
+-----------------+-------------+------+-----+---------+-------+
| Field           | Type        | Null | Key | Default | Extra |
+-----------------+-------------+------+-----+---------+-------+
| id              | int         | NO   | PRI | NULL    |       |
| department_name | varchar(25) | YES  |     | NULL    |       |
| avg_salary      | float       | YES  |     | NULL    |       |
| num_of_people   | int         | YES  |     | 0       |       |
| location        | varchar(20) | YES  |     | NULL    |       |
+-----------------+-------------+------+-----+---------+-------+
 5 rows in set (0.00 sec)

mysql>

  5. 删除字段
    删除字段是将数据表中的某个字段从表中移除：ALTER TABLE <表名> DROP <字段名>;。

    mysql> desc department;
+-----------------+-------------+------+-----+---------+-------+
| Field           | Type        | Null | Key | Default | Extra |
+-----------------+-------------+------+-----+---------+-------+
| id              | int         | NO   | PRI | NULL    |       |
| department_name | varchar(25) | YES  |     | NULL    |       |
| avg_salary      | float       | YES  |     | NULL    |       |
| num_of_people   | int         | YES  |     | 0       |       |
| location        | varchar(20) | YES  |     | NULL    |       |
+-----------------+-------------+------+-----+---------+-------+
 5 rows in set (0.00 sec)

mysql> alter table department drop avg_salary;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc department;
+-----------------+-------------+------+-----+---------+-------+
| Field           | Type        | Null | Key | Default | Extra |
+-----------------+-------------+------+-----+---------+-------+
| id              | int         | NO   | PRI | NULL    |       |
| department_name | varchar(25) | YES  |     | NULL    |       |
| num_of_people   | int         | YES  |     | 0       |       |
| location        | varchar(20) | YES  |     | NULL    |       |
+-----------------+-------------+------+-----+---------+-------+
 4 rows in set (0.00 sec)

mysql>

  6. 修改字段的排列位置
    对于一个数据表来说，在创建的时候，字段在表中的排列顺序就已经确定了，但表的结构并不是完全不可以改变的，可以通过ALTER TABLE来改变表中字段的相对位置。语法格式如下：ALTER TABLE <表名> MODIFY <字段1> <数据类型> FIRST|AFTER <字段2>;。

    mysql> desc department;
+-----------------+-------------+------+-----+---------+-------+
| Field           | Type        | Null | Key | Default | Extra |
+-----------------+-------------+------+-----+---------+-------+
| id              | int         | NO   | PRI | NULL    |       |
| department_name | varchar(25) | YES  |     | NULL    |       |
| num_of_people   | int         | YES  |     | 0       |       |
| location        | varchar(20) | YES  |     | NULL    |       |
+-----------------+-------------+------+-----+---------+-------+
 4 rows in set (0.00 sec)

mysql> alter table department modify department_name varchar(250) after num_of_people;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc department;
+-----------------+--------------+------+-----+---------+-------+
| Field           | Type         | Null | Key | Default | Extra |
+-----------------+--------------+------+-----+---------+-------+
| id              | int          | NO   | PRI | NULL    |       |
| num_of_people   | int          | YES  |     | 0       |       |
| department_name | varchar(250) | YES  |     | NULL    |       |
| location        | varchar(20)  | YES  |     | NULL    |       |
+-----------------+--------------+------+-----+---------+-------+
 4 rows in set (0.00 sec)

mysql>

  7. 更改表的存储引擎
    我们可以根据自己的需要，选择不同的引擎，甚至可以为每一张表选择不同的存储引擎。MySQL中主要的存储引擎有MyISAM、InnoDB、MEMORY（HEAP）、BDB、FEDERATED等。可以使用SHOW ENGINES;语句查看系统支持的存储引擎。更改储存引擎的语法：ALTER TABLE <表名> ENGINE=<更改后的存储引擎名>;。

    mysql> show engines;
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| Engine             | Support | Comment                                                        | Transactions | XA   | Savepoints |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| ARCHIVE            | YES     | Archive storage engine                                         | NO           | NO   | NO         |
| BLACKHOLE          | YES     | /dev/null storage engine (anything you write to it disappears) | NO           | NO   | NO         |
| MRG_MYISAM         | YES     | Collection of identical MyISAM tables                          | NO           | NO   | NO         |
| FEDERATED          | NO      | Federated MySQL storage engine                                 | NULL         | NULL | NULL       |
| MyISAM             | YES     | MyISAM storage engine                                          | NO           | NO   | NO         |
| PERFORMANCE_SCHEMA | YES     | Performance Schema                                             | NO           | NO   | NO         |
| InnoDB             | DEFAULT | Supports transactions, row-level locking, and foreign keys     | YES          | YES  | YES        |
| MEMORY             | YES     | Hash based, stored in memory, useful for temporary tables      | NO           | NO   | NO         |
| CSV                | YES     | CSV storage engine                                             | NO           | NO   | NO         |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
 9 rows in set (0.00 sec)

mysql> alter table user_login_info engine=memory;
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql>

  8. 删除表的外键约束
    对于数据库中定义的外键，如果不再需要，可以将其删除。外键一旦删除，就会解除主表和从表间的关联关系。语法：ALTER TABLE <表名> DROP FOREIGN KEY <外键约束名>

    mysql> desc user_info;
+---------------+--------------+------+-----+---------+-------+
| Field         | Type         | Null | Key | Default | Extra |
+---------------+--------------+------+-----+---------+-------+
| id            | int          | YES  |     | NULL    |       |
| username      | varchar(200) | YES  |     | NULL    |       |
| department_id | int          | YES  | MUL | NULL    |       |
| age           | int          | YES  |     | NULL    |       |
+---------------+--------------+------+-----+---------+-------+
 4 rows in set (0.00 sec)

mysql> alter table user_info drop foreign key fk_user_department;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table user_info drop foreign key fk_user_department;
ERROR 1091 (42000): Can't DROP 'fk_user_department'; check that column/key exists

删除数据表

  1. 删除没有被关联的表
    语法：DROP TABLE [IF EXISTS]表1, 表2,…表n;。

    mysql> show tables;
+-----------------+
| Tables_in_ronie |
+-----------------+
| department      |
| tmp             |
| user_info       |
| user_login_info |
+-----------------+
 4 rows in set (0.00 sec)

mysql> drop table if exists tmp;
Query OK, 0 rows affected (0.00 sec)

mysql> show tables;
+-----------------+
| Tables_in_ronie |
+-----------------+
| department      |
| user_info       |
| user_login_info |
+-----------------+
 3 rows in set (0.01 sec)

  2. 删除被其他表关联的主表
    在数据表之间存在外键关联的情况下，如果直接删除父表，结果会显示失败，原因是直接删除将破坏表的参照完整性。如果必须要删除，可以先删除与它关联的子表，再删除父表，只是这样就同时删除了两个表中的数据。有的情况下可能要保留子表，这时若要单独删除父表，只需将关联的表的外键约束条件取消，然后就可以删除父表。

    mysql> create table main
 -> (
 -> id int(10) primary key,
 -> main_name varchar(25)
 -> );
Query OK, 0 rows affected, 1 warning (0.01 sec)

mysql> create table follow
 -> (
 -> id int(10) primary key,
 -> follow_name varchar(25),
 -> main_id int(10),
 -> constraint fk_follow_main foreign key (main_id) references main (id)
 -> );
Query OK, 0 rows affected, 2 warnings (0.02 sec)

mysql> show tables;
+-----------------+
| Tables_in_ronie |
+-----------------+
| department      |
| follow          |
| main            |
| user_info       |
| user_login_info |
+-----------------+
 5 rows in set (0.00 sec)

mysql> drop main;
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 'main' at line 1

mysql> show create table main \G;
*************************** 1. row ***************************
    Table: main
Create Table: CREATE TABLE `main` (
  `id` int NOT NULL,
  `main_name` varchar(25) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
 1 row in set (0.00 sec)

ERROR: 
No query specified

mysql> show create table follow \G;
*************************** 1. row ***************************
    Table: follow
Create Table: CREATE TABLE `follow` (
  `id` int NOT NULL,
  `follow_name` varchar(25) DEFAULT NULL,
  `main_id` int DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fk_follow_main` (`main_id`),
  CONSTRAINT `fk_follow_main` FOREIGN KEY (`main_id`) REFERENCES `main` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
 1 row in set (0.00 sec)

ERROR: 
No query specified

mysql> drop main;
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 'main' at line 1

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

mysql> drop table follow;
Query OK, 0 rows affected (0.01 sec)

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