Database Management System (DBMS) / SQL(STRUCTURED QUERY LANGUAGE)

Data Definition Language (DDL)

DDL refers to the SQL commands used to define, modify, and remove the structure of database objects such as tables, schemas, and indexes.

🧱 CREATE – Create New Database Objects

Used to create tables, databases, views, indexes, etc.

📌 Example:

-- Step 1: Create the students table
CREATE TABLE students (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  grade CHAR(2)
);
-- Step 2: Insert some sample student data
INSERT INTO students (id, name, grade) VALUES 
(1, 'Swastika', 'A+'),
(2, 'Anya', 'B'),
(3, 'Rahul', 'A'),
(4, 'Neha', 'C');

-- Step 3: View the student records
SELECT * FROM students;

🔸 Creates a table named students with 4 columns

🧰 ALTER – Modify an Existing Table

Used to add, modify, or drop columns and constraints.

📌 Add a column:

-- Step 1: Create the students table
-- Step 2: Insert some sample student data

ALTER TABLE students ADD email VARCHAR(100);

-- Step 3: View the student records

☘️What this does:

  • Adds a new column named email to the existing students table.

  • The column type is VARCHAR(100) → it can store email addresses up to 100 characters.

📌 Modify a column:

-- Step 1: Create the students table
-- Step 2: Insert some sample student data

-- Step 3: Modify grade column type
ALTER TABLE students 
ALTER COLUMN grade VARCHAR(5);

INSERT INTO students (id, name, grade) VALUES
(5, 'Ishaan', 'FAIL');

-- Step 5: View the student records

📌 What this does:

  • Changes the data type of the grade column from CHAR(2) (or whatever it was) to VARCHAR(5).

✅ Why do this?

  • Allows storing longer grade values like 'A++', 'FAIL', 'B+1', etc.

🗑️ DROP – Delete Database Objects

Permanently deletes a table, database, or view

📌 Drop a table:

DROP TABLE students;

🧹 TRUNCATE – Delete All Records from a Table

Removes all rows from a table, but keeps its structure

📌 Example:

TRUNCATE TABLE students;

⚡ Faster than DELETE

❌ Cannot be rolled back in many databases

✅ Summary of DDL Commands

Command

Purpose

🧱 CREATE

Define a new table or database

🧰 ALTER

Modify structure of an existing table

🗑️ DROP

Delete table or database completely

🧹 TRUNCATE

Remove all data, keep table structure