Top 30 SQL Developer Interview Questions & Answers with Real-Time Scenarios – 2025

SQL Developers are essential for handling databases, writing queries, and optimizing performance. This list features the top 30 SQL interview questions with real-world scenarios tailored to 2025 job roles.

Basic Concepts

• 1. What is SQL and why is it used?
  SQL (Structured Query Language) is used to communicate with and manipulate relational databases.
• 2. What are the different types of SQL statements?
  - DDL (CREATE, DROP)
  - DML (INSERT, UPDATE, DELETE)
  - DCL (GRANT, REVOKE)
  - TCL (COMMIT, ROLLBACK)
• 3. Difference between WHERE and HAVING?
  WHERE filters rows before grouping; HAVING filters groups after aggregation.
• 4. What are constraints in SQL?
  Rules like PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, and CHECK to enforce data integrity.
• 5. What is normalization?
  Process of organizing data to reduce redundancy and improve data integrity.

Joins & Subqueries

• 6. What are the types of joins?
  INNER, LEFT, RIGHT, FULL OUTER, CROSS JOIN.
• 7. Write a query to fetch employee names and their department names.
  SELECT e.name, d.dept_name FROM employees e JOIN departments d ON e.dept_id = d.id;
• 8. What is a subquery?
  A query nested within another query. Used for filtering or calculating intermediate values.
• 9. How to get the second highest salary?
  SELECT MAX(salary) FROM employees WHERE salary < (SELECT MAX(salary) FROM employees);
• 10. What’s a correlated subquery?
  A subquery that uses values from the outer query and is re-evaluated for each row.

Real-Time Scenarios

• 11. How would you find duplicate records in a customer table?
  SELECT name, COUNT(*) FROM customers GROUP BY name HAVING COUNT(*) > 1;
• 12. How do you update a column with values from another table?
  UPDATE orders o SET o.status = 'Shipped' FROM shipments s WHERE o.id = s.order_id;
• 13. Scenario: Find customers who haven’t placed any orders.
  SELECT * FROM customers WHERE id NOT IN (SELECT customer_id FROM orders);
• 14. Scenario: Get top 3 products sold by revenue.
  SELECT product_id, SUM(amount) FROM sales GROUP BY product_id ORDER BY SUM(amount) DESC LIMIT 3;
• 15. Scenario: Get employees who joined in the last 30 days.
  SELECT * FROM employees WHERE join_date >= CURRENT_DATE - INTERVAL '30 days';

Performance & Optimization

• 16. What is an index?
  An index speeds up data retrieval by creating a pointer structure to rows.
• 17. How do you identify slow-running queries?
  By using EXPLAIN PLAN or query profiling tools provided by the database system.
• 18. What is the difference between clustered and non-clustered indexes?
  Clustered index stores actual data with the index. Non-clustered stores a pointer to the data.
• 19. When should you avoid using SELECT *?
  To improve performance and prevent fetching unnecessary columns.
• 20. How to avoid SQL injection?
  Use parameterized queries or stored procedures instead of dynamic SQL.

Advanced SQL

• 21. What are window functions?
  Functions that perform calculations across a set of rows related to the current row (e.g., RANK, ROW_NUMBER, LAG).
• 22. What’s a CTE?
  Common Table Expression – a temporary result set defined with WITH clause.
• 23. Difference between DELETE and TRUNCATE?
  DELETE removes rows one at a time (can be rolled back), TRUNCATE is faster but can’t be rolled back in some DBs.
• 24. How do you handle large data migration in production?
  By batching data in chunks, using transactions, and scheduling during low-traffic windows.
• 25. What’s a stored procedure and when would you use it?
  A reusable block of SQL code stored in the DB. Used for consistency, performance, and reusability.

Culture Fit & Miscellaneous

• 26. How do you collaborate with developers and analysts?
  By participating in sprint reviews, refining data models together, and discussing query logic openly.
• 27. How do you test the accuracy of your queries?
  By using known test data, cross-checking with business rules, and validating totals with sample rows.
• 28. How do you ensure data integrity?
  Use constraints, proper normalization, and avoid manual data overrides.
• 29. Describe a time you solved a complex data issue.
  I identified performance issues caused by nested subqueries and replaced them with indexed joins, improving speed 5x.
• 30. How do you stay updated with SQL practices?
  By reading technical blogs, exploring DB vendor updates, and working on real-time data problems.