Introduction
If you’re short on time and need to pass an interview that tests your database skills, this guide gives clear, practice-ready answers to the Top 30 Most Common SQL Query Interview Questions And Answers You Should Prepare For. The SQL Query Interview Questions across entry and mid-level roles test fundamentals, joins, aggregations, window functions, and performance thinking—areas you can master with deliberate practice and targeted examples. Read the concise Q&A below, practice the queries, and use the takeaways to shape answers in live interviews.
What are SQL Query Interview Questions?
SQL Query Interview Questions are technical prompts that test your ability to write, optimize, and reason about SQL code.
Employers use these questions to evaluate how you model data, join tables, aggregate results, and tune performance for real workloads; they range from beginner SELECTs to window functions and optimization. Examples here mirror common industry patterns and include practice tips from high-quality resources to help you answer clearly in interviews. Takeaway: Focus on correctness first, then explain complexity and trade-offs.
How should you prepare for SQL Query Interview Questions?
Prepare by combining concept review, hands-on query practice, and timed mock interviews.
Start with fundamentals—joins, GROUP BY, and subqueries—then practice window functions and query tuning on real datasets; use curated lists and practice platforms to simulate interview pressure. According to LambdaTest’s interview prep guide, structured practice and reviewing execution plans dramatically improve readiness. Takeaway: Build a repeatable practice routine that includes writing queries and explaining them aloud.
Top 30 SQL Query Interview Questions and Answers
These 30 concise Q&A pairs mirror what data analyst, data engineer, and data scientist candidates frequently face in interviews. Use them to practice writing solutions and explaining your approach.
Technical Fundamentals
Q: What is SQL?
A: Structured Query Language used to manage and query relational databases like MySQL, PostgreSQL, and SQL Server.
Q: What is a primary key?
A: A unique identifier for table rows that cannot be NULL and enforces entity integrity.
Q: What is the difference between WHERE and HAVING?
A: WHERE filters rows before aggregation; HAVING filters groups after GROUP BY.
Q: What are the common types of JOINs?
A: INNER, LEFT (LEFT OUTER), RIGHT (RIGHT OUTER), FULL OUTER, and CROSS JOIN.
Q: How does an INNER JOIN differ from a LEFT JOIN?
A: INNER JOIN returns only matching rows; LEFT JOIN returns all left-table rows plus matches or NULLs.
Q: What is a self-join?
A: Joining a table to itself to compare rows within the same table, using aliases to distinguish instances.
Q: Subquery vs JOIN: when to use each?
A: Use JOINs for combining related rows across tables; use subqueries for filtering or computing derived values when joins complicate logic.
Q: What is a correlated subquery?
A: A subquery that refers to columns from the outer query, evaluated per outer-row, often less efficient than joins.
Aggregations, Grouping, and Sorting
Q: Name common aggregate functions.
A: COUNT, SUM, AVG, MIN, MAX used with GROUP BY to summarize data.
Q: GROUP BY vs ORDER BY: what’s the difference?
A: GROUP BY aggregates rows into groups; ORDER BY sorts the final result set.
Q: How to find the second highest salary in a table?
A: Use window functions (ROW_NUMBER or RANK) or a subquery with MAX excluding the top salary.
Q: Explain window functions and give an example.
A: Functions like ROW_NUMBER(), RANK(), SUM() OVER(PARTITION BY ...) compute values across rows related to the current row without collapsing groups.
Q: RANK vs DENSE_RANK: how do they differ?
A: RANK leaves gaps after ties; DENSE_RANK does not create gaps in ranking sequence.
Q: How do you aggregate only the top N values per group?
A: Use ROWNUMBER() PARTITION BY group ORDER BY metric and filter WHERE rownum <= N.
Q: When would you use HAVING instead of WHERE with aggregates?
A: Use HAVING to filter groups after aggregation, e.g., HAVING COUNT(*) > 10.
Advanced Queries and Optimization
Q: What are Common Table Expressions (CTEs) and when to use them?
A: WITH clauses that create named temporary result sets for readability or stepwise logic; useful for recursive queries.
Q: Temp tables vs CTEs: pros and cons?
A: Temp tables persist for the session and can be indexed; CTEs are inline and improve readability but may re-evaluate.
Q: What is an index and why use one?
A: A data structure (B-tree, hash) that accelerates searches; reduces full table scans at the cost of write overhead.
Q: How do you read an execution plan?
A: Look for scans vs seeks, join order, estimated vs actual rows, and expensive operators to identify bottlenecks.
Q: Explain normalization and denormalization briefly.
A: Normalization reduces redundancy across tables; denormalization duplicates data to improve read performance for analytics.
Q: What are transactions and the ACID properties?
A: Transactions ensure Atomicity, Consistency, Isolation, Durability for reliable multi-step operations.
Q: What are common transaction isolation levels?
A: Read Uncommitted, Read Committed, Repeatable Read, Serializable—tradeoffs between concurrency and consistency.
Q: How do you handle deadlocks?
A: Detect via DB logs, design shorter transactions, access resources in consistent order, and implement retry logic.
Q: Give a simple approach to SQL optimization.
A: Profile with EXPLAIN, add appropriate indexes, avoid SELECT *, reduce nested loops, and rewrite heavy subqueries.
Q: What is the purpose of the EXPLAIN statement?
A: Shows the database’s planned execution strategy so you can identify scans, joins, and costly operations.
Practical and Scenario-Based
Q: How do you treat NULL values in comparisons?
A: Use IS NULL / IS NOT NULL; be mindful that NULL != NULL and that functions may need COALESCE to supply defaults.
Q: How to concatenate strings across rows?
A: Use STRINGAGG, GROUPCONCAT, or FOR XML PATH/STRING_AGG equivalents depending on the RDBMS.
Q: How do you find duplicate rows?
A: GROUP BY key_columns HAVING COUNT(*) > 1 to list duplicate keys and counts.
Q: How to perform an UPSERT (insert or update)?
A: Use MERGE, INSERT ... ON CONFLICT/ON DUPLICATE KEY UPDATE, or database-specific upsert syntax.
Q: What is a pivot operation?
A: Converts row values into columns using PIVOT, conditional aggregation, or CASE statements for reporting.
Q: How to retrieve the top N rows per group?
A: Use ROW_NUMBER() OVER (PARTITION BY group ORDER BY metric DESC) and filter by row number.
Q: How would you handle large import performance?
A: Bulk load utilities, disable indexes during load, batch commits, and tune logging for the import process.
Q: How do you explain your approach if you can't fully solve a problem in an interview?
A: Describe assumptions, outline steps, provide partial solutions, and show how you'd test and optimize—clarity matters.
How Verve AI Interview Copilot Can Help You With This
Verve AI Interview Copilot gives live, contextual feedback as you answer SQL Query Interview Questions and structures responses with clear stepwise reasoning. Verve AI Interview Copilot also helps you practice window functions, joins, and optimization by suggesting concise explanations and query sketches during mock runs. Use Verve AI Interview Copilot to rehearse answers, polish explanations, and reduce interview anxiety with targeted prompts and scoring.
What Are the Most Common Questions About This Topic
Q: Can Verve AI help with behavioral interviews?
A: Yes. It applies STAR and CAR frameworks to guide real-time answers.
Q: Are these SQL Query Interview Questions suitable for data science roles?
A: Yes. They cover joins, aggregations, window functions, and optimization relevant to data roles.
Q: Where can I practice these SQL Query Interview Questions online?
A: Use platforms like the ones linked from Stratascratch and InterviewBit for interactive practice.
Q: How often should I practice SQL Query Interview Questions?
A: Short daily sessions (30–60 minutes) with review and mock interviews boost retention.
Q: Can I use these SQL Query Interview Questions in a timed mock?
A: Yes; time yourself and explain answers aloud to mirror interview conditions.
Conclusion
Practicing these SQL Query Interview Questions builds correctness, clarity, and the ability to explain trade-offs under pressure. Focus on writing clean queries, reading execution plans, and structuring answers that show how you verify and optimize results. Try Verve AI Interview Copilot to feel confident and prepared for every interview.
References: For curated question lists and practice resources, see Stratascratch, GeeksforGeeks, DataLemur, InterviewBit, and LambdaTest community guidance.
