Top 50 SQL Interview Questions

1. What is SQL?
2. Differentiate between SQL and NoSQL databases.
3. What are the different types of SQL commands?
4. Explain the difference between WHERE and HAVING clauses.
5. Write a SQL query to find the second highest salary in a table.
6. What is a JOIN? Explain different types of JOINs.
7. How do you optimize slow-performing SQL queries?
8. What is a primary key? What is a foreign key?
9. What are indexes? Explain clustered and non-clustered indexes.
10. Write a SQL query to fetch the top 5 records from a table.
11. What is a subquery? Give an example.
12. Explain the concept of normalization.
13. What is denormalization? When is it used?
14. Describe transactions and their properties (ACID).
15. What is a stored procedure?
16. How do you handle NULL values in SQL?
17. Explain the difference between UNION and UNION ALL.
18. What are views? How are they useful?
19. What is a trigger? Give use cases.
20. How do you perform aggregate functions in SQL?
21. What is data partitioning?
22. How do you find duplicates in a table?
23. What is the difference between DELETE and TRUNCATE?
24. Explain window functions with examples.
25. What is the difference between correlated and non-correlated subqueries?
26. How do you enforce data integrity?
27. What are CTEs (Common Table Expressions)?
28. Explain EXISTS and NOT EXISTS operators.
29. How do SQL constraints work?
30. What is an execution plan? How do you use it?
31. Describe how to handle errors in SQL.
32. What are temporary tables?
33. Explain the difference between CHAR and VARCHAR.
34. How do you perform pagination in SQL?
35. What is a composite key?
36. How do you convert data types in SQL?
37. Explain locking and isolation levels in SQL.
38. How do you write recursive queries?
39. What are the advantages of using prepared statements?
40. How to debug SQL queries?
41. Differentiate between OLTP and OLAP databases.
42. What is schema in SQL?
43. How do you implement many-to-many relationships in SQL?
44. What is query optimization?
45. How do you handle large datasets in SQL?
46. Explain the difference between CROSS JOIN and INNER JOIN.
47. What is a materialized view?
48. How do you backup and restore a database?
49. Explain how indexing can degrade performance.
50. Can you write a query to find employees with no managers?

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