MySQL DDL syntax

MySQL DDL syntax

Introduction to DDL

MySQL DDL (Data Definition Language) is a language used to define and manage database structures. It includes statements that create, modify, and delete databases, tables, views, indexes, and other database objects. The importance of DDL syntax is as follows:

1. Database structure definition: DDL statements are used to create and define the structure of database objects, such as creating tables and defining fields, data types, constraints and indexes, etc. Properly defining the database structure is the basis for ensuring data integrity and consistency.

2. Database object management: DDL statements can modify and delete database objects. Through DDL statements, you can modify the table structure, add or delete fields, constraints and indexes. This allows the database structure to be flexibly managed and adjusted according to business needs to adapt to changing data management needs.

3. Database security and permission control: DDL statements are used to manage permissions and access control of database users. Through DDL statements, you can create and manage users, assign and revoke permissions, and ensure database security and data protection.

4. Database performance optimization: DDL statements can optimize database performance. Through correct table design, index creation and optimized DDL statements, query performance can be improved, storage space usage can be reduced, and data read and write operations can be optimized.

5. Database version control and migration: DDL statements record changes to the database structure. Through version control tools, DDL statements can be stored as scripts to facilitate version management and migration of database structures, ensuring the consistency and traceability of the database in different environments.

In short, the importance of DDL syntax in MySQL is reflected in its ability to define, manage and adjust database structures, maintain database security, optimize database performance, and support database version control and migration. The correct use and understanding of DDL statements is crucial for database managers and developers. They need to properly plan and maintain the database structure to ensure the accuracy, efficiency and security of data storage, access and update.

DDL syntax classification

DDL (Data Definition Language) grammar mainly includes the following categories:

1. Create database object:

   • CREATE DATABASE: Create a database.
   • CREATE TABLE: Create a table.
   • CREATE INDEX: Create an index.
   • CREATE VIEW: Create a view.
   • CREATE TRIGGER: Create a trigger.

2. Modify database objects:

   1. ALTER {ADD|MODIFY|REMOVE|RENAME|CHANGE}
   2. ALTER TABLE: Modify the table structure, such as adding, modifying and deleting columns, constraints, etc.
   3. ALTER INDEX: Modify the index.
   4. ALTER VIEW: Modify the view.
   5. ALTER TRIGGER: Modify the trigger.

3. Delete database objects:

   • DROP DATABASE: Delete the database.
   • DROP TABLE: Delete the table.
   • DROP INDEX: Delete the index.
   • DROP VIEW: Delete the view.
   • DROP TRIGGER: Delete a trigger.

These are common DDL syntax categories. According to different syntax classifications, different operations can be performed, such as creating and modifying database objects, managing permissions, transaction management, etc. Understanding and being familiar with the classification of DDL syntax will help you better use and manage database objects and achieve the goals of data definition and management.

DDL statement prototype

The DDL syntax for creating database objects is as follows:

1. CREATE DATABASE: Create a database

   • Syntax prototype:

   CREATE DATABASE [IF NOT EXISTS] database_name
   [CHARACTER SET charset_name]
   [COLLATE collation_name]
   

   • Parameter Description:
     • database_name: The name of the database to be created.
     • IF NOT EXISTS(Optional): If the specified database already exists, no action is performed.
     • CHARACTER SET charset_name(Optional): Specify the character set of the database (e.g. utf8).
     • COLLATE collation_name(Optional): Specify the collation rules of the database.

2. CREATE TABLE: create table

   • Syntax prototype:

   CREATE TABLE table_name
   (
     column1 datatype constraints,
     column2 datatype constraints,
     ...
   )
   [ENGINE = engine_name]
   [DEFAULT CHARACTER SET charset_name]
   [COLLATE collation_name]
   

   • Parameter Description:
     • table_name: The name of the table to be created.
     • column1, column2, ...: Column definitions of the table, including column names, data types and related constraints.
     • ENGINE = engine_name(Optional): Specify the storage engine to use (such as InnoDB).
     • DEFAULT CHARACTER SET charset_name(Optional): Specifies the default character set for the table.
     • COLLATE collation_name(Optional): Specify the default collation rules for the table.

3. CREATE INDEX: Create an index

   • Syntax prototype:

   CREATE INDEX index_name
   ON table_name (column1, column2, ...)
   

   • Parameter Description:
     • index_name: The name of the index to create.
     • table_name: On which table to create the index.
     • column1, column2, ...: Columns to be included in the index.

4. CREATE VIEW: Create a view

   • Syntax prototype:

   CREATE VIEW view_name AS
   SELECT column1, column2, ...
   FROM table_name
   WHERE condition
   

   • Parameter Description:
     • view_name: The name of the view to create.
     • column1, column2, ...: Columns included in the view.
     • table_name: Which table to select data from.
     • WHERE condition(optional): Conditions to limit selection.

5. CREATE TRIGGER: Create a trigger

   • Syntax prototype:

   CREATE TRIGGER trigger_name
   {BEFORE | AFTER} {INSERT | UPDATE | DELETE}
   ON table_name
   FOR EACH ROW
   BEGIN
     -- 触发器的操作逻辑
   END;
   

   • Parameter Description:
     • trigger_name: The name of the trigger to create.
     • {BEFORE | AFTER}: Specify the trigger to execute before or after the relevant event.
     • {INSERT | UPDATE | DELETE}:Specifies which operation the trigger is related to.
     • table_name: Table related to triggers.
     • FOR EACH ROW: Triggered for each row.

The above is the DDL syntax and parameter description for creating database objects. Based on specific needs and logic, objects such as databases, tables, indexes, views, and triggers can be created using appropriate syntax, and their properties and behaviors can be defined.

Modify database objects:

• The relevant syntax for modifying database objects is as follows:

  1. ALTER {ADD|MODIFY|REMOVE|RENAME|CHANGE}：

  • ALTER ADD：

    • Syntax prototype:ALTER TABLE table_name ADD column_name datatype [constraints];
    • Parameter Description:
      • table_name: The name of the table to be modified.
      • column_name: The name of the column to be added.
      • datatype: The data type of the column.
      • [constraints]: Column constraints, such as NOT NULL, DEFAULT, etc.
    • Meaning: Add a new column to an existing table.

  • ALTER MODIFY：

    • Syntax prototype:ALTER TABLE table_name MODIFY column_name datatype [constraints];
    • Parameter Description:
      • table_name: The name of the table to be modified.
      • column_name: The name of the column to be modified.
      • datatype: The new data type of the column.
      • [constraints]: Column constraints, such as NOT NULL, DEFAULT, etc.
    • Meaning: Modify the data type of columns in the existing table.

  • ALTER REMOVE：

    • Syntax prototype:ALTER TABLE table_name DROP column_name;
    • Parameter Description:
      • table_name: The name of the table to be modified.
      • column_name: The name of the column to be deleted.
    • Meaning: Delete the specified column from the existing table.

  • ALTER RENAME：

    • Syntax prototype:ALTER TABLE table_name RENAME old_column_name TO new_column_name;
    • Parameter Description:
      • table_name: The name of the table to be modified.
      • old_column_name:Original column name.
      • new_column_name: New column name.
    • Meaning: Modify the column names in the existing table.

  • ALTER CHANGE：

    • Syntax prototype:ALTER TABLE table_name CHANGE old_column_name new_column_name datatype [constraints];
    • Parameter Description:
      • table_name: The name of the table to be modified.
      • old_column_name:Original column name.
      • new_column_name: New column name.
      • datatype: The data type of the column.
      • [constraints]: Column constraints, such as NOT NULL, DEFAULT, etc.
    • Meaning: Modify the column names in the existing table, and change the data type and constraints of the columns at the same time.
  2. ALTER TABLE: used to modify the table structure, such as adding, modifying and deleting columns, constraints, etc.
  • Syntax prototype:ALTER TABLE table_name {ADD|MODIFY|DROP} column_name datatype [constraints];
  • Parameter Description:
    • table_name: The name of the table to be modified.
    • ADD: Add column.
    • MODIFY: Modify the data type of the column.
    • DROP: Delete a column.
    • column_name: The name of the column to be added, modified or deleted.
    • datatype: The data type of the column.
    • [constraints]: Column constraints, such as NOT NULL, DEFAULT, etc.
  • Meaning: You can add, modify, and delete table columns through the ALTER TABLE statement.
  3. ALTER INDEX: Modify the index.
  • Syntax prototype:ALTER TABLE table_name ALTER INDEX index_name {RENAME TO new_index_name | SET index_type};
  • Parameter Description:
    • table_name: The name of the table to be modified.
    • index_name: The name of the index to be modified.
    • RENAME TO new_index_name: Modify the index name.
    • SET index_type: Modify the index type.
  • Meaning: The ALTER INDEX statement can modify the index of the table, and the name or type of the index can be modified.
  4. ALTER VIEW: Modify the view.
  • Syntax prototype:ALTER VIEW view_name AS new_view_definition;
  • Parameter Description:
    • view_name: The name of the view to be modified.
    • new_view_definition: New view definition.
  • Meaning: The definition of the view can be modified through the ALTER VIEW statement, and the query logic of the view can be updated.
  5. ALTER TRIGGER: Modify the trigger.
  • Syntax prototype:ALTER TRIGGER trigger_name {BEFORE | AFTER} {INSERT | UPDATE | DELETE} ON table_name FOR EACH ROW BEGIN ... END;
  • Parameter Description:
    • trigger_name: The name of the trigger to be modified.
    • {BEFORE | AFTER}: When the trigger is fired (before or after the relevant event).
    • {INSERT | UPDATE | DELETE}: Which operation the trigger is related to.
    • table_name: The table name associated with the trigger.
    • BEGIN ... END: Modify the operation logic of the trigger.
  • Meaning: The definition and operation logic of the trigger can be modified through the ALTER TRIGGER statement.

Relevant syntax for deleting database objects

1. DROP DATABASE: Delete the database.

   • Syntax prototype:DROP DATABASE database_name;
   • Parameter Description:
     • database_name: The name of the database to be deleted.
   • Meaning: This statement is used to permanently delete the specified database and all associated tables, views, indexes, triggers and other data objects.

2. DROP TABLE: Delete the table.

   • Syntax prototype:DROP TABLE table_name;
   • Parameter Description:
     • table_name: The name of the table to be deleted.
   • Meaning: This statement is used to permanently delete the specified table and all its data and constraints.

3. DROP INDEX: Delete the index.

   • Syntax prototype:DROP INDEX index_name ON table_name;
   • Parameter Description:
     • index_name: The name of the index to be deleted.
     • table_name: The name of the table to which the index belongs.
   • Meaning: This statement is used to permanently delete the index in the specified table.

4. DROP VIEW: Delete the view.

   • Syntax prototype:DROP VIEW view_name;
   • Parameter Description:
     • view_name: The name of the view to be deleted.
   • Meaning: This statement is used to permanently delete the specified view.

5. DROP TRIGGER: Delete a trigger.

   • Syntax prototype:DROP TRIGGER trigger_name ON table_name;
   • Parameter Description:
     • trigger_name: The name of the trigger to be deleted.
     • table_name: The name of the table to which the trigger belongs.
   • Meaning: This statement is used to permanently delete the specified trigger.

DDL statement specification

1. Keyword case: As mentioned before, keywords in SQL are usually expressed in uppercase letters. It is recommended that keywords in DDL statements be unified into uppercase letters to increase readability.
2. Naming convention for identifiers: Similarly, when naming database objects in DDL statements, you should follow certain naming conventions, use meaningful and descriptive names, and avoid using reserved words or special characters to avoid grammatical errors. This rule applies to CREATE, ALTER and DROP statements in DDL, etc.
3. Indentation and line breaks: To improve readability, DDL statements should be grouped and aligned using appropriate indentation and line breaks. For example, in a CREATE TABLE statement, use indentation to align column definitions to make the code structure clear.
4. Use of comments: Add comments to DDL statements to explain and illustrate the code to increase the readability of the code. Comments should be clear and describe the main operation and purpose.
5. Error handling and rollback: Before executing DDL statements, exception handling mechanisms, such as using TRY-CATCH blocks, should be considered to handle potential error conditions and perform rollback operations as needed.
6. Security and permissions: When DDL statements involve the creation, modification, and deletion of database objects, it is necessary to ensure that only authorized users can perform these operations, and carry out necessary permission controls to ensure data security.

DDL SQL injection method

DDL (Data Definition Language) SQL injection refers to an attacker injecting malicious DDL SQL code into the user input of an application to perform unauthorized database structure modification operations. Unlike DML SQL injection, the goal of DDL SQL injection attacks is to modify the structure of the database rather than the content of the data. Here are some common DDL SQL injection methods:

1. Delete table injection: An attacker deletes tables in the database by injecting malicious code into the DROP TABLE statement. For example, set the input parameter to: '; DROP TABLE table_name;–, so that the entire table is deleted.

2. Create table injection: An attacker can exploit the injection vulnerability in the CREATE TABLE statement to create a new table in the database to perform malicious operations. For example, setting the input parameter to: '); CREATE TABLE malicious_table (id INT);– will create a malicious table.

3. Modify table injection: An attacker can inject malicious code in the ALTER TABLE statement to modify the structure of the database table. They can add, delete or modify columns, constraints, etc. For example, setting the input parameter to: '; ALTER TABLE table_name ADD COLUMN malicious_column VARCHAR(100);– will add a malicious column to the table.

4. Inject other DDL statements: DDL SQL injection can use other DDL statements to perform unauthorized database structure modification operations. For example, an attacker can use EXECUTE IMMEDIATE or other dynamic statement execution functions in the application to execute malicious DDL statements.

Methods to prevent DDL SQL injection are similar to those to prevent DML SQL injection, including using parameter binding, input validation and sanitization, restricting database user permissions, logging and exception handling, etc. In addition, the application's database operation permissions should also be restricted to ensure that the application can only perform legitimate DDL operations, and regular security reviews and vulnerability scans should be conducted to ensure the security of the application.

Summarize

DDL statements are statements used to define and manage database structures and objects, including creating, modifying, and deleting databases, tables, views, indexes, etc. Mastering DDL statements is crucial for database design, optimization and maintenance.