SQL > Data Definition Language (DDL)

Introduction

Data Definition Language (DDL) is a subset of SQL used to define and manage database structures. DDL commands do not manipulate data but instead define the schema of the database.

Key DDL Commands:

• CREATE - Used to create a new database object (table, user, database, etc.).
• ALTER - Used to modify an existing database object.
• DROP - Used to delete a database object.

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CREATE

Purpose:

The CREATE command is used to define new tables, databases, users, and other database objects.

Syntax:

CREATE TABLE table_name (
    column1 datatype constraints,
    column2 datatype constraints,
    ...
);

Example:

CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    FirstName VARCHAR(50),
    LastName VARCHAR(50),
    Age INT,
    Salary DECIMAL(10,2)
);

Creating a Database:

CREATE DATABASE CompanyDB;

Creating a User:

CREATE USER 'newuser'@'localhost' IDENTIFIED BY 'password';

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ALTER

Purpose:

The ALTER command is used to modify the structure of an existing database object.

Syntax:

ALTER TABLE table_name
ADD COLUMN column_name datatype;

Example:

Adding a new column:

ALTER TABLE Employees
ADD COLUMN Department VARCHAR(50);

Modifying a column:

ALTER TABLE Employees
MODIFY COLUMN Age SMALLINT;

Dropping a column:

ALTER TABLE Employees
DROP COLUMN Salary;

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DROP

Purpose:

The DROP command is used to delete database objects permanently.

Syntax:

DROP TABLE table_name;

Example:

Dropping a table:

DROP TABLE Employees;

Dropping a database:

DROP DATABASE CompanyDB;

Dropping a user:

DROP USER 'newuser'@'localhost';

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Summary

Command	Description
CREATE	Creates a new database object (table, database, user).
ALTER	Modifies an existing database object.
DROP	Deletes a database object permanently.

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