Data Definition Language (DDL) Commands:

Data Definition Language (DDL) commands in SQL are used to define, modify, and manage the structure of database objects like tables, indexes, and views. Here are some common DDL commands:

CREATE TABLE:

Creates a new table with specified columns, data types, constraints, and other attributes.

CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    FirstName VARCHAR(50),
    LastName VARCHAR(50),
    Department VARCHAR(100),
    Salary DECIMAL(10, 2)
);

ALTER TABLE:

Modifies an existing table, adding or dropping columns, constraints, or other properties.

-- Add a new column
ALTER TABLE Employees
ADD Email VARCHAR(100);

-- Drop a column
ALTER TABLE Employees
DROP COLUMN Department;

DROP TABLE:

Removes an existing table and all its data permanently.

DROP TABLE Employees;

CREATE INDEX:

Creates an index on one or more columns of a table, which enhances data retrieval performance.

CREATE INDEX idx_LastName ON Employees(LastName);

DROP INDEX:

Removes an existing index from a table.

DROP INDEX idx_LastName ON Employees;

CREATE VIEW:

Creates a virtual table based on the result of a SELECT query, providing a convenient way to access complex data.

CREATE VIEW HighSalaryEmployees AS
SELECT * FROM Employees WHERE Salary > 50000;

DROP VIEW:

Removes an existing view.

DROP VIEW HighSalaryEmployees;

CREATE DATABASE:

Creates a new database.

CREATE DATABASE CompanyDB;

ALTER DATABASE:

Modifies properties of an existing database.

ALTER DATABASE CompanyDB
SET RECOVERY SIMPLE; -- Change recovery model

Remember that DDL commands have a significant impact on the structure of your database. Use them with caution, as they can result in permanent changes to your data. Also, keep in mind that syntax and specific options may vary depending on the database management system you’re using. Always refer to your DBMS documentation for accurate details.