📌 What is GRANT in PostgreSQL?

In PostgreSQL, the GRANT command is used to give permissions (privileges) to users (roles) on database objects like tables, schemas, or databases.

👉 In simple words: GRANT = Give access to someone

🎯 Why do we use GRANT?

We use GRANT to:

• 🔐 Control who can access what

• 👨‍💻 Allow users to perform actions like:

  • SELECT (read data)
  • INSERT (add data)
  • UPDATE (modify data)
  • DELETE (remove data)

• 🛡️ Improve database security

• 🧑‍🤝‍🧑 Support multi-user environments

🧾 Basic Syntax

GRANT privilege(s)
ON object_name
TO role_name;

🧩 Common Privileges

✅ Example

Step 1: Create a user (role)

CREATE ROLE trainee WITH LOGIN PASSWORD '1234';

Step 2: Grant access to a table

GRANT SELECT ON customer TO trainee;

👉 Now trainee can only view data from customer table.

🔄 Multiple Privileges Example

GRANT SELECT, INSERT, UPDATE
ON customer
TO trainee;

👉 Now user can:

• Read
• Add
• Modify data

🌐 Grant on Entire Database

GRANT CONNECT ON DATABASE dvdrental TO trainee;

👉 Allows user to connect to the dvdrental database.

🧠 Important Notes

• You must be owner or superuser to grant permissions.
• Permissions can be revoked using REVOKE.
• Roles can be users or groups.

🧪 Practical Lab Exercise (Beginner Friendly)

🎯 Objective:

Learn how to control access using GRANT.

🔹 Task 1: Create a Role

CREATE ROLE student1 WITH LOGIN PASSWORD 'pass123';

🔹 Task 2: Grant Database Access

GRANT CONNECT ON DATABASE dvdrental TO student1;

🔹 Task 3: Grant Read Access

GRANT SELECT ON film TO student1;

👉 Test: Login as student1 and run:

SELECT * FROM film;

🔹 Task 4: Try Restricted Action

INSERT INTO film(title) VALUES ('Test Movie');

❌ This should fail (no INSERT permission)

🔹 Task 5: Grant INSERT Permission

GRANT INSERT ON film TO student1;

👉 Try INSERT again → ✅ Should work

🔹 Task 6: Grant All Permissions

GRANT ALL ON film TO student1;

🔹 Challenge Task 💡

1. Create another role viewer
2. Allow only SELECT on actor table
3. Block all other operations

🧾 Summary

• GRANT = Give permission
• Used for security & access control
• Works on tables, databases, schemas
• Helps manage multiple users safely

Below is a complete step-by-step real-world university system scenario showing:

✔ Database creation ✔ Table design ✔ Role creation ✔ GRANT usage (security layer) ✔ Testing access like a real system

Using PostgreSQL

🎓 University System with GRANT (Step-by-Step Lab)

🏁 Goal

We will build a small University Database System where:

• Students can only view data
• Teachers can manage marks
• Admin controls everything using GRANT

ERD Diagram for University Database System

🧱 STEP 1: Create Database

CREATE DATABASE university_db;

👉 Now connect to database:

\c university_db

🏗️ STEP 2: Create Main Tables

📘 2.1 Students Table

CREATE TABLE students (
    student_id SERIAL PRIMARY KEY,
    name VARCHAR(100),
    department VARCHAR(50),
    email VARCHAR(100)
);

📘 2.2 Courses Table

CREATE TABLE courses (
    course_id SERIAL PRIMARY KEY,
    course_name VARCHAR(100),
    credit_hours INT
);

📘 2.3 Enrollments Table

CREATE TABLE enrollments (
    enroll_id SERIAL PRIMARY KEY,
    student_id INT REFERENCES students(student_id),
    course_id INT REFERENCES courses(course_id)
);

📘 2.4 Marks Table

CREATE TABLE marks (
    mark_id SERIAL PRIMARY KEY,
    student_id INT REFERENCES students(student_id),
    course_id INT REFERENCES courses(course_id),
    marks INT
);

👥 STEP 3: Create Roles (Users)

🎓 Student Role

CREATE ROLE student WITH LOGIN PASSWORD '123';

👨‍🏫 Teacher Role

CREATE ROLE teacher WITH LOGIN PASSWORD '123';

🧑‍💼 Admin Role

CREATE ROLE admin WITH LOGIN PASSWORD 'admin123';

🔐 STEP 4: Grant Database Access

Allow connection to database

GRANT CONNECT ON DATABASE university_db TO student;
GRANT CONNECT ON DATABASE university_db TO teacher;
GRANT CONNECT ON DATABASE university_db TO admin;

📊 STEP 5: GRANT Permissions (Core Security)

🎓 5.1 Student Permissions (Read Only)

GRANT SELECT ON students TO student;
GRANT SELECT ON courses TO student;
GRANT SELECT ON enrollments TO student;
GRANT SELECT ON marks TO student;

👉 Students can ONLY VIEW data

❌ Cannot insert/update/delete

👨‍🏫 5.2 Teacher Permissions

GRANT SELECT ON students TO teacher;
GRANT SELECT ON courses TO teacher;
GRANT SELECT ON enrollments TO teacher;

Teachers can manage marks:

GRANT SELECT, INSERT, UPDATE ON marks TO teacher;

🧑‍💼 5.3 Admin Permissions (Full Control)

GRANT ALL PRIVILEGES ON students TO admin;
GRANT ALL PRIVILEGES ON courses TO admin;
GRANT ALL PRIVILEGES ON enrollments TO admin;
GRANT ALL PRIVILEGES ON marks TO admin;

🧪 STEP 6: Insert Sample Data (Admin Role)

INSERT INTO students (name, department, email)
VALUES 
('Ali Khan', 'CS', 'ali@uni.edu'),
('Sara Ahmed', 'IT', 'sara@uni.edu');

INSERT INTO courses (course_name, credit_hours)
VALUES 
('Database Systems', 3),
('Web Development', 4);

👨‍🎓 STEP 7: Testing Role Access

🔐 Login as Student

SELECT * FROM students;

✔ Allowed

❌ Try Insert (Should FAIL)

INSERT INTO students(name, department, email)
VALUES ('Test Student', 'CS', 'test@uni.edu');

👉 ERROR: permission denied

👨‍🏫 Login as Teacher

✔ View students

SELECT * FROM students;

✔ Insert marks

INSERT INTO marks(student_id, course_id, marks)
VALUES (1, 1, 85);

✔ Update marks

UPDATE marks
SET marks = 90
WHERE student_id = 1;

🧑‍💼 Login as Admin

Full control:

DELETE FROM students WHERE student_id = 2;

✔ Works

🧠 STEP 8: How GRANT Works (Concept)

Think of it like a University Gate System:

🔐 STEP 9: Security Logic (Important)

✔ GRANT = give permission ✔ REVOKE = remove permission ✔ Roles = users or groups

👉 Without GRANT: Users cannot even see tables

📌 FINAL SUMMARY

In this real-world university system:

✔ Database created ✔ Tables designed ✔ Roles created ✔ Permissions assigned using GRANT ✔ Access controlled by role type