Structured Query Language (SQL) continues to be the foundation of data management and database systems. Whether you are applying for your first developer role, preparing for a data analyst position, or moving up as a database administrator, having a clear understanding of SQL interview questions and answers is critical.

This guide has been carefully prepared to walk you through 85 commonly asked SQL interview questions and answers for 2025. The explanations are simple, practical, and easy to grasp.

Introduction to SQL Interviews

Most technical interviews test more than just syntax knowledge. Employers expect you to understand concepts such as query optimization, joins, indexing, and handling real-world database challenges. SQL interview questions and answers often move from basic definitions to scenario-based tasks.

Basic SQL Interview Questions and Answers

1. What is SQL?

SQL stands for Structured Query Language, which is used to communicate with databases. It allows users to insert, update, delete, and retrieve data efficiently.

2. What are tables in SQL?

Tables are organized collections of rows and columns where data is stored. Each column represents a field, and each row represents a record.

3. What is a primary key?

A primary key uniquely identifies each record in a table. It cannot contain duplicate or null values.

4. What is a foreign key?

A foreign key establishes a relationship between two tables by referencing the primary key of another table.

5. What is the difference between SQL and MySQL?

SQL is the language, while MySQL is a database management system that uses SQL as its querying language.

Intermediate SQL Interview Questions and Answers

6. What are joins in SQL?

Joins are used to combine data from multiple tables based on related columns.

7. Explain different types of joins.

• INNER JOIN – returns rows with matching values in both tables.
  
• LEFT JOIN – returns all rows from the left table and matched rows from the right.
  
• RIGHT JOIN – returns all rows from the right table and matched rows from the left.
  
• FULL JOIN – returns all rows when there is a match in one of the tables.
  

8. What is a subquery?

A subquery is a query nested inside another SQL query. It helps in filtering, calculations, or creating conditions dynamically.

9. What is the difference between WHERE and HAVING?

• WHERE is used to filter rows before grouping.
  
• HAVING is used to filter groups after aggregation.
  

10. What are indexes in SQL?

Indexes speed up data retrieval by creating a separate data structure that helps in quick lookups.

Advanced SQL Interview Questions and Answers

11. What is normalization?

Normalization is the process of organizing data to reduce redundancy and improve data integrity.

12. What are different normal forms?

• 1NF: Eliminate repeating groups.
  
• 2NF: Eliminate partial dependency.
  
• 3NF: Eliminate transitive dependency.
  
• BCNF: Stronger version of 3NF.
  

13. What is denormalization?

Denormalization is the process of combining tables to improve read performance, often at the cost of redundancy.

14. What is a stored procedure?

A stored procedure is a group of SQL statements stored in the database that can be executed as a single unit.

15. What is a trigger in SQL?

A trigger is an automatic action performed by the database in response to certain events, such as inserts, updates, or deletes.

Practical SQL Interview Questions and Answers

16. How do you find the second highest salary in a table?

SELECT MAX(salary) 

FROM employees 

WHERE salary < (SELECT MAX(salary) FROM employees);

17. How do you count unique values in a column?

SELECT COUNT(DISTINCT column_name) FROM table_name;

18. How do you find duplicate records in a table?

SELECT column_name, COUNT(*) 

FROM table_name 

GROUP BY column_name 

HAVING COUNT(*) > 1;

19. How do you delete duplicate records but keep one?

DELETE FROM employees 

WHERE rowid NOT IN (

SELECT MIN(rowid) 

FROM employees 

 GROUP BY employee_name

20. How do you retrieve top N rows in SQL?

SELECT * FROM employees ORDER BY salary DESC LIMIT N;

Scenario-Based SQL Interview Questions and Answers

21. What happens if a foreign key constraint is violated?

The database prevents the action to maintain referential integrity.

22. How can you improve query performance?

• Use indexes wisely.
  
• Avoid unnecessary subqueries.
  
• Retrieve only required columns.
  
• Use joins instead of multiple subqueries.
  

23. What is a transaction in SQL?

A transaction is a unit of work that is either completed fully or not executed at all.

24. What are ACID properties?

• Atomicity – all steps succeed or none do.
  
• Consistency – database remains valid.
  
• Isolation – transactions do not interfere with each other.
  
• Durability – changes are permanent once committed.
  

25. How can you roll back a transaction?

Use the ROLLBACK command before committing changes.

Complex SQL Interview Questions and Answers

26. What is a window function?

A window function performs calculations across a set of rows related to the current row, without collapsing rows into a single output.

27. What is the difference between RANK, DENSE_RANK, and ROW_NUMBER?

• RANK: Leaves gaps after ties.
  
• DENSE_RANK: No gaps after ties.
  
• ROW_NUMBER: Assigns a unique number regardless of ties.
  

28. What is the difference between UNION and UNION ALL?

• UNION removes duplicates.
  
• UNION ALL includes all records, even duplicates.
  

29. What is the difference between DELETE, TRUNCATE, and DROP?

• DELETE removes rows but keeps structure.
  
• TRUNCATE removes all rows quickly but keeps structure.
  
• DROP removes the table entirely.
  

30. How do you handle NULL values in SQL?

Use functions like ISNULL(), COALESCE(), or conditional checks.

Additional Frequently Asked SQL Interview Questions and Answers

From 31 to 85, you may encounter variations such as:

• Difference between clustered and non-clustered indexes.
  
• Use of EXISTS vs IN.
  
• Purpose of self-joins.
  
• How to handle recursive queries.
  
• Ways to pivot and unpivot data.
  
• Writing queries for date and time calculations.
  
• Understanding CTE (Common Table Expressions).
  
• How to secure SQL queries against injection.
  
• Advantages of using views.
  
• Role of constraints like UNIQUE, CHECK, and DEFAULT.
  

Each of these areas can come up depending on the role you are applying for.

Final Thoughts

SQL remains one of the most tested skills during interviews for technical roles. Preparing for SQL interview questions and answers not only sharpens your database knowledge but also makes you more confident in handling real-world problems.

If you practice writing queries daily, experiment with sample databases, and review advanced topics like indexing, optimization, and transactions, you will be ready for any level of SQL interview in 2025.