DDL

📘 What is DDL (Data Definition Language)?

DDL (Data Definition Language) is the part of SQL used to define and manage the structure of a database — tables, columns, indexes, constraints, schemas, etc.

If a database were a building, DDL would be the architecture and construction tools — deciding how many rooms (tables) there are, their size (columns), and rules (constraints).

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🔧 Key DDL Commands

Command	What It Does
CREATE	Creates new database objects like tables, views, indexes, schemas
ALTER	Modifies the structure of existing objects (add/remove column, change type)
DROP	Deletes an object permanently (e.g., table, view, index)
TRUNCATE	Deletes all rows from a table quickly (but keeps the structure intact)
RENAME	Renames an object like a table or column

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🛠️ Example Usage:

1. CREATE

CREATE TABLE users (
  id INT PRIMARY KEY,
  name VARCHAR(100),
  email VARCHAR(255) UNIQUE
);

Creates a users table with 3 columns and constraints.

2. ALTER

ALTER TABLE users ADD COLUMN created_at TIMESTAMP;

Adds a new column to the users table.

3. DROP

DROP TABLE users;

Permanently deletes the users table and all its data.

4. TRUNCATE

TRUNCATE TABLE users;

Quickly deletes all data in users, but keeps the table.

5. RENAME

ALTER TABLE users RENAME TO customers;

Renames the users table to customers.

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⚠️ Important Notes:

Concept	Explanation
DDL is auto-committed	Changes are immediate and permanent — no need to COMMIT
Cannot be rolled back (usually)	Once a DROP is done, the data is gone unless backups exist
Impacts the schema	DDL affects how the data is stored, not the data itself

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🧪 Real-World Analogy:

• A developer uses DDL to create and design the database structure before the app even has users.

• Changing a column name or data type later is also DDL.

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🧠 Interview Line:

“DDL defines the structure of the database using commands like CREATE, ALTER, and DROP. It controls how data is stored, not the data itself, and its changes are auto-committed.”