SQL remains the backbone of modern data-driven systems — and it’s showing no signs of slowing down. If you’re preparing for SQL interviews in 2025, especially with 3–5 years of experience, you’ll need more than just basic SQL queries to succeed.
Companies today are looking for professionals who can optimise queries, handle real-world datasets, and think critically about database performance and scalability.
This guide covers the top 20 must-know advanced SQL interview questions and answers — with practical examples, pro tips, and real-world scenarios you can expect in your next technical round. Let’s dive in!
Top 20 Advanced SQL Interview Questions and Answers
1. What is the difference between clustered and non-clustered indexes?
Answer:
- Clustered Index: Organises data rows physically in order based on the index key. Only one clustered index per table.
- Non-Clustered Index: Separate from the actual data rows, containing pointers. A table can have multiple non-clustered indexes.
2. What is the purpose of a CTE (Common Table Expression)?
Answer:
A CTE makes complex queries easier to manage and read. It’s useful for breaking down large queries into understandable parts and is essential for recursive queries.
Related Read: SQL Interview Questions and Answers for Data Analyst
3. Explain the ACID properties of a transaction.
Answer:
- Atomicity: All operations succeed or none do.
- Consistency: The database remains in a valid state.
- Isolation: Transactions do not interfere with each other.
- Durability: Changes persist even after a system failure.
4. How does indexing affect performance?
Answer:
Indexes speed up data retrieval but can slow down write operations. They are essential for optimizing read-heavy applications but should be used judiciously.
5. What is normalization? Why is it important?
Answer:
Normalization structures a database to minimize redundancy and improve data integrity, typically following normal forms like 1NF, 2NF, and 3NF.
6. What is denormalization?
Answer:
Denormalization introduces redundancy to enhance read performance, often used in reporting systems where read speed is prioritized over update performance.
7. How do SQL Joins work? Name different types.
Answer:
- INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN, CROSS JOIN — all combine records from two or more tables based on logical relationships.
8. What’s the difference between DELETE and TRUNCATE?
Answer:
- DELETE: Removes rows individually and can be filtered.
- TRUNCATE: Removes all rows quickly without individual-row logging.
Related Read: SQL Interview Questions and Answers for Testers
9. What is a correlated subquery?
Answer:
A subquery that depends on the outer query for its values — evaluated repeatedly for each row processed by the outer query.
10. Explain window functions.
Answer:
Functions like ROW_NUMBER(), RANK(), DENSE_RANK(), LEAD(), LAG() allow computations across sets of rows related to the current query row without collapsing the result set.
11. How do you optimize SQL queries?
Answer:
Use selective columns, proper indexes, avoid complex joins when unnecessary, leverage CTEs, and study execution plans.
12. What is a deadlock? How can you prevent it?
Answer:
A deadlock happens when two transactions block each other. Prevent it by acquiring locks in the same order, keeping transactions short, and using lock timeout strategies.
13. What are materialized views, and how do they differ from regular views?
Answer:
Materialized views store query results physically, unlike standard views that are re-executed on demand.
14. How do you handle NULL values in SQL?
Answer:
Use IS NULL, COALESCE(), or NULLIF() to properly handle NULLs without introducing logical errors.
Related Read: Top 50 SQL Query Interview Questions and Answers
15. What’s the difference between UNION and UNION ALL?
Answer:
- UNION removes duplicates.
- UNION ALL keeps all records (including duplicates).
16. How does indexing impact JOIN performance?
Answer:
Indexes on join columns significantly reduce lookup time and improve query performance by preventing full table scans.
17. What is a surrogate key?
Answer:
A surrogate key is an artificial, unique identifier (often numeric) assigned to each record, independent of the actual data.
18. How do stored procedures differ from functions?
Answer:
Stored procedures can modify data and manage transactions; functions are intended to return a value and cannot change data.
19. Explain database sharding.
Answer:
Sharding splits a database horizontally to distribute the data across multiple machines, improving performance and scalability.
20. What are SQL triggers, and when should you use them carefully?
Answer:
Triggers automatically execute in response to table events. Overusing triggers can make systems harder to debug and maintain.
Related Read: Advanced SQL Server Questions and Answers
Frequently Asked Questions (FAQs)
Q1. What types of SQL questions are asked for 3–5 years experienced professionals?
Expect advanced topics like query optimization, handling large datasets, transaction control, complex JOINs, CTEs, and indexing strategies.
Q2. How important is it to know indexing strategies for SQL interviews?
Extremely important. Good indexing improves performance and showcases your ability to work with real-world data challenges.
Q3. What are common mistakes candidates make in advanced SQL interviews?
Using SELECT *, ignoring execution plans, mishandling NULLs, overcomplicating queries, and poor transaction management are typical pitfalls.
Q4. Should I focus more on normalization or denormalization for technical interviews?
Both are important — know how to normalize for data integrity and when to denormalize for performance optimization.
Q5. Are triggers and stored procedures still relevant topics for SQL interviews in 2025?
Yes. They’re critical for implementing complex business rules and maintaining database integrity.
Q6. How can I approach SQL query optimization questions during interviews?
Focus on identifying bottlenecks, suggesting indexes, rewriting queries for efficiency, and explaining your optimization steps clearly.
Q7. What advanced JOIN concepts should I be familiar with for SQL interviews?
Understand self-joins, anti-joins, semi-joins, and how JOINs behave with large datasets for real-world SQL challenges.
Q8. What’s the role of transaction isolation levels in SQL interviews?
Isolation levels define how transactions interact — understanding when and why to adjust them is key to optimizing concurrency and integrity.
Conclusion
SQL remains one of the most critical skills in the tech industry.
Mastering these top 20 advanced SQL interview questions will not only prepare you for your next opportunity but also give you a deeper understanding of how databases truly work behind the scenes.
Whether you’re targeting SQL developer jobs or transitioning from SQL fresher jobs into mid-level or senior roles, sharpening your advanced SQL skills is one of the smartest moves you can make in 2025.
