🗃️ SQL – Interview-Centric Topics

This section includes all core SQL topics frequently asked in interviews. Learn about different types of joins, subqueries, indexing, normalization forms, CTEs, window functions, stored procedures, and SQL injection prevention. Each topic includes multiple-choice questions, code-based Q&A, and practical use cases.

• 1. SQL Joins Explained: INNER, LEFT, RIGHT, FULL
• 2. SQL Subqueries and Nested Queries Explained
• 3. SQL Aggregate Functions: COUNT, SUM, AVG, MIN, MAX
• 4. Understanding SQL GROUP BY and ORDER BY
• 5. Difference Between DELETE, TRUNCATE and DROP
• 6. SQL Views: Use Cases and Performance Tips
• 7. Common Table Expressions (CTE) in SQL
• 8. SQL Window Functions and OVER Clause
• 9. SQL Indexes: Clustered vs Non-Clustered
• 10. SQL Normalization and Denormalization
• 11. SQL Constraints: NOT NULL, UNIQUE, CHECK, DEFAULT
• 12. SQL Injection and How to Prevent It
• 13. Primary Key vs Foreign Key in SQL
• 14. Data Types in SQL: INT, VARCHAR, DATE and more
• 15. How to Use CASE Statement in SQL
• 16. Writing Complex SQL Queries Step-by-Step
• 17. Self Join in SQL with Example
• 18. SQL Union vs Union All: Key Differences
• 19. Difference Between CHAR and VARCHAR in SQL
• 20. SQL Joins with Multiple Tables
• 21. Performance Tuning Tips in SQL Queries
• 22. NULL Handling in SQL: IS NULL vs IS NOT NULL
• 23. SQL EXISTS vs IN vs JOIN Performance
• 24. How to Write Stored Procedures in SQL
• 25. Triggers in SQL: Before vs After
• 26. Working with Temporary Tables in SQL
• 27. Difference Between INNER JOIN and CROSS JOIN
• 28. SQL Ranking Functions: RANK, DENSE_RANK, ROW_NUMBER
• 29. Index Scan vs Index Seek in SQL Server
• 30. Use of Aliases in SQL Queries
• 31. Locking and Deadlocks in SQL
• 32. Common SQL Interview Query Patterns
• 33. Recursive Queries with CTE in SQL
• 34. How to Optimize SELECT Statements
• 35. Use of DISTINCT and TOP Clauses in SQL
• 36. Writing Parameterized Queries in SQL
• 37. Working with JSON Data in SQL Server / PostgreSQL
• 38. Using PIVOT and UNPIVOT in SQL Server
• 39. SQL Functions vs Procedures: Key Differences
• 40. How to Export Data Using SQL Queries
• 41. SQL Data Import Techniques and Tools
• 42. Role of Database Schema in SQL Design
• 43. Advanced SQL Joins: Anti Join and Semi Join
• 44. Detecting and Removing Duplicates in SQL
• 45. SQL Interview Challenges with Real Examples
• 46. Common Mistakes to Avoid in SQL Queries
• 47. What is the difference between INNER JOIN and LEFT JOIN?
• 48. How does GROUP BY work in SQL and when should it be used?
• 49. Explain the difference between WHERE and HAVING clauses.
• 50. What is a primary key and how is it different from a unique key?
• 51. What are the different types of joins in SQL?
• 52. What is a subquery and where can it be used?
• 53. How do you prevent SQL injection in queries?
• 54. What is the difference between DELETE and TRUNCATE commands?
• 55. How does the RANK() function differ from DENSE_RANK()?
• 56. What is a window function in SQL and how is it different from aggregate functions?
• 57. Explain the purpose and syntax of the CASE statement in SQL.
• 58. What is normalization and why is it important?
• 59. Describe different normal forms in database normalization.
• 60. What are the benefits and drawbacks of denormalization?
• 61. What is a foreign key and how does it enforce referential integrity?
• 62. How is a composite key different from a primary key?
• 63. What is the difference between a clustered index and a non-clustered index?
• 64. Explain the difference between a temporary table and a CTE.
• 65. What is a correlated subquery and how does it differ from a regular subquery?
• 66. How can you find duplicate records in a SQL table?
• 67. What is the use of COALESCE() and how is it different from ISNULL()?
• 68. How do you handle NULL values in SQL comparisons?
• 69. What is a transaction and what are its ACID properties?
• 70. How do you implement transaction control using COMMIT and ROLLBACK?
• 71. What are isolation levels in SQL and how do they affect concurrency?
• 72. What is a deadlock in SQL and how can it be resolved or prevented?
• 73. How do triggers work in SQL and when should they be used?
• 74. What is the difference between BEFORE and AFTER triggers?
• 75. How can you optimize a slow-running SQL query?
• 76. What tools or commands can you use to analyze query performance?
• 77. How does indexing improve performance and when should it be used?
• 78. What are the drawbacks of over-indexing a table?
• 79. How can you update records from one table based on another table?
• 80. What is the difference between UNION and UNION ALL?
• 81. How does the MERGE statement work in SQL Server?
• 82. How do you write a recursive CTE and what are its use cases?
• 83. What is a materialized view and how does it differ from a regular view?
• 84. What are the best practices for writing efficient SQL queries?
• 85. How do you handle errors in SQL procedures or scripts?
• 86. What is the difference between a stored procedure and a function in SQL?
• 87. How can you create a pivot table using SQL queries?
• 88. How do you transpose rows to columns in SQL?
• 89. What are JSON functions in SQL and how do you query JSON data?
• 90. How do you export query results to a CSV or external file using SQL?
• 91. How can you schedule recurring queries or reports in SQL Server or PostgreSQL?
• 92. What is the role of the INFORMATION_SCHEMA in SQL?
• 93. How do you detect and fix orphaned records in a relational database?