Top SQL Interview Questions

Curated list of 100 essential SQL interview questions spanning beginner, intermediate, and advanced levels—all sourced from leading resources:

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🔰 Beginner

1. What is SQL?
   
2. What is a database / DBMS / RDBMS?
   
3. SQL vs MySQL vs NoSQL?
   
4. SQL command categories ?
   
5. CREATE vs ALTER vs DROP?
   
6. INSERT, UPDATE, DELETE syntax?
   
7. WHERE vs HAVING?
   
8. GROUP BY vs ORDER BY?
   
9. DISTINCT usage?
   
10. UNION vs UNION ALL?
    
11. INTERSECT and MINUS?
    
12. LIKE and wildcards?
    
13. NULL handling and COALESCE?
    
14. CHAR vs VARCHAR vs TEXT?
    
15. What are constraints ?
    
16. Primary key vs Unique key?
    
17. Foreign key and referential integrity?
    
18. What is a view?
    
19. What is a subquery?
    
20. Correlated vs non‑correlated subquery?
    
21. Aggregate functions ?
    
22. ORDER BY clause?
    
23. Basic operators: arithmetic, comparison, logical?
    
24. What is a cursor?
    
25. What is a trigger?
    
26. Views/types ?
    
27. SQL data types?
    
28. BETWEEN and IN operators?
    
29. CROSS JOIN vs INNER JOIN vs OUTER JOIN types?
    
30. SELF JOIN?
    
31. Pagination ?
    
32. Creating tables like another?
    
33. Pattern matching?
    
34. SQL injection prevention?
    
35. Referential integrity?
    

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🧩 Intermediate

1. What is a Common Table Expression (CTE)?
   
2. What is a recursive CTE? Give a use case.
   
3. Explain window (analytic) functions (e.g., ROW_NUMBER, RANK, DENSE_RANK).
   
4. How would you calculate running totals or moving averages using window functions?
   
5. What is the difference between CROSS JOIN, INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN?
   
6. EXISTS vs IN vs JOIN — when should you use each?
   
7. How do you calculate gaps in a sequence (e.g. missing IDs)?
   
8. How do you rank items within groups—e.g., top 3 salaries per department?
   
9. Handling NULLs in aggregation—how does COALESCE or ISNULL help?
   
10. How to implement FULL OUTER JOIN in MySQL (which lacks native support)?
    
11. Use of LAG() and LEAD() functions — what problems do they solve?
    
12. What is a correlated subquery vs non‑correlated subquery? Provide examples.
    
13. Difference between WHERE and HAVING clauses.
    
14. Why use GROUP BY? Give an example.
    
15. What is conditional aggregation and how does it help pivot data without PIVOT syntax?
    
16. What are PIVOT and UNPIVOT operations?
    
17. Explain dynamic SQL. Pros and cons?
    
18. Compare temporary tables vs table variables (e.g., in SQL Server).
    
19. What are stored procedures vs user-defined functions?
    
20. What are triggers? When is AFTER INSERT trigger useful?
    
21. Explain index types—clustered vs non‑clustered vs composite vs unique.
    
22. What is index fragmentation? How do you rebuild or reorganize indexes?
    
23. Describe how execution plans work and how EXPLAIN (or equivalent) is used.
    
24. What is parameter sniffing and how can it affect performance?
    
25. Explain implicit conversions and their impact on index usage.
    
26. How to analyze wait statistics or blocking/deadlocks in a database?
    
27. Explain isolation levels: Read Uncommitted, Read Committed, Repeatable Read, Serializable.
    
28. When and why would you denormalize a database schema?
    
29. Vertical vs horizontal partitioning (sharding)—what's the difference?
    
30. What are indexed views or materialized views? How do they enhance performance?
    
31. Difference between DELETE, TRUNCATE, and DROP (again focusing on transaction logging).
    
32. What is the MERGE (UPSERT) statement and when to use it?
    
33. How do you handle recursive/hierarchical data using CONNECT BY vs recursive CTEs?
    
34. Explain CROSS APPLY vs OUTER APPLY (T‑SQL / SQL Server lateral join equivalents).
    
35. What are statistics in SQL Server (or your RDBMS)? How do they affect the optimizer?
    
    

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🚀 Advanced

1. Transactions & ACID properties?
   
2. Deadlocks, locks, and prevention?
   
3. Stored Procedures vs Functions?
   
4. Temporary tables vs Table variables?
   
5. Dynamic SQL?
   
6. MERGE statement?
   
7. Materialized/indexed views?
   
8. Query execution plan and EXPLAIN?
   
9. Index types: clustered vs non-clustered vs unique vs composite?
   
10. Index fragmentation & maintenance?
    
11. Parameter sniffing in queries?
    
12. Implicit conversions & performance?
    
13. Wait statistics & blocking?
    
14. Partitioning and sharding?
    
15. Row-level security?
    
16. Error handling ?
    
17. Recursive/hierarchical queries using CONNECT BY or CTEs?
    
18. Full-text search?
    
19. CROSS APPLY vs OUTER APPLY?
    
20. Statistics in SQL Server?
    
21. Query plan memory grants?
    
22. ETL staging tables and data pipelines?
    
23. OLTP vs OLAP vs Data Warehousing schemas ?
    
24. Surrogate keys vs natural keys?
    
25. Database normalization forms ?
    
26. Denormalization scenarios?
    
27. Column vs row store and in-memory tables?
    
28. Data lake & SQL integration ?
    
29. Microservices and polyglot persistence?
    
30. Internationalization, encryption, SQL security best practices?