SQL Interview Questions

1. How Does the SELECT Statement Work in SQL?

2. How Does the WHERE Clause Work in SQL?

3. INNER JOIN vs LEFT JOIN in SQL

4. How Does GROUP BY Work in SQL?

5. HAVING vs WHERE in SQL

6. What are Aggregate Functions in SQL?

7. What Does the DISTINCT Keyword Do in SQL?

8. What are Subqueries in SQL?

9. What is a Correlated Subquery in SQL?

10. UNION vs UNION ALL in SQL

11. What are Window Functions in SQL?

12. RANK vs DENSE_RANK vs ROW_NUMBER in SQL

13. What are Common Table Expressions (CTEs) in SQL?

14. How Does the CASE Statement Work in SQL?

15. EXISTS vs IN in SQL

16. What is a Self Join in SQL?

17. How Do You Find the Second Highest Salary in SQL?

18. How Do You Find Duplicate Records in SQL?

19. How Do You Delete Duplicate Rows in SQL?

20. How Do LIMIT and OFFSET Work in SQL?

21. How Does the LIKE Operator Work in SQL?

22. How Do You Handle NULL Values in SQL?

23. How Do You Find the Nth Highest Salary in SQL?

24. How Do You UPDATE with a JOIN in SQL?

25. What are Date Functions in SQL?

26. What are String Functions in SQL?

27. How Do You Pivot Data in SQL?

28. How Do COALESCE and NULLIF Work in SQL?

29. What is a CROSS JOIN in SQL?

30. How Does ORDER BY Work in SQL?

31. What is a JOIN?

32. Explain Types of SQL Joins

33. Difference Between WHERE and HAVING?

34. What is Normalization?

35. What is an Index?

36. What is a Primary Key vs Foreign Key?

37. What is a Subquery?

38. What is a CTE?

39. What Is the Difference Between UNION and UNION ALL?

40. What Is a View in SQL?

41. What Is the Difference Between a Clustered and a Non-Clustered Index?

42. What Is a Transaction in SQL?

43. What Is the Difference Between GROUP BY and ORDER BY?

44. What Is the Difference Between CHAR and VARCHAR?

45. What Is Denormalization in Database Design?

46. How Do You Find the Second Highest Salary in SQL?

47. How Do You Read an Execution Plan and Work Out Why the Optimiser Chose a Scan Instead of an Index Seek?

48. How Does Index Selectivity Work, and How Do You Decide the Column Order in a Composite Index?

49. What Is a SARGable Predicate, and Which Common WHERE Clauses Silently Disable Index Use?

50. How Do Window Function Frame Clauses Work, and Why Does ROWS Differ From RANGE?

51. How Does a Recursive CTE Work, and How Do You Stop One Running Forever on Cyclic Data?

52. When Would You Use EXISTS Rather Than a Join or a Correlated Subquery, and How Do They Differ in Practice?

53. How Do NULLs Behave in Aggregates, Joins and NOT IN, and Where Do They Silently Change Results?

54. What Anomalies Does Each Transaction Isolation Level Permit, and How Do You Choose One?

55. How Do You Diagnose a Deadlock, and What Is Lock Escalation?

56. Why Does a MERGE or Check-Then-Insert Upsert Race Under Concurrency, and How Do You Make It Safe?

57. Why Does OFFSET Pagination Get Slower on Later Pages, and How Does Keyset Pagination Fix It?

58. A Query Was Fast Yesterday and Slow Today With No Code Change — How Do Statistics and Parameter Sniffing Explain That?

59. When Does Table Partitioning Actually Help, and What Stops Partition Pruning From Working?

60. How Do You Add a Column, an Index or a Constraint to a Large Live Table Without Downtime?

61. How Do You Delete or Backfill Millions of Rows Without Stalling Production?