Difference between Authentication and Authorization? (Auth verifies identity, e.g., login; Authorization checks permissions, e.g., role-based access. What is JWT? (JSON Web Token: Stateless token with header, payload, signature for secure API auth in MERN apps.) ​ How to secure passwords? (Hash with bcrypt/Argon2, add salt, enforce strong rules, rate-limit logins.) at’s the difference between DELETE, TRUNCATE, and DROP? (DELETE removes rows conditionally; TRUNCATE deletes all without logging; DROP removes the table structure.) ​ Write a query to find the second highest salary: SELECT MAX(salary) FROM Employee WHERE salary < (SELECT MAX(salary) FROM Employee); ​ ​ How do you find duplicates? SELECT column, COUNT(*) FROM table GROUP BY column HAVING COUNT(*) > 1; ​ Common Authentication Questions Difference between Authentication and Authorization? (Auth verifies identity, e.g., login; Authorization checks permissions, e.g., role-based access.) ​ What is JWT? (JSON Web Token: Stateless token with header, payload, signature for secure API auth in MERN apps.) ​ How to secure passwords? (Hash with bcrypt/Argon2, add salt, enforce strong rules, rate-limit logins.) ​ Practice Tips Use GROUP BY with aggregates: SELECT dept, AVG(salary) FROM Employee GROUP BY dept; for department averages. ​ For joins: Practice INNER, LEFT, SELF joins on employee-manager tables. ​ Implement OAuth2 in Spring Boot or Node.js for microservices, tying to your cloud certs. ​ Related Common SQL queries for user authentication systems How to store passwords securely in a database Designing tables for users sessions and tokens SQL queries to verify email and activate accounts Preventing SQL injection in authentication queries