Top 20 SQL Interview Questions for 2025

SQL is a non-negotiable skill for developers, analysts, and engineers in 2025. Whether you're applying for backend roles or data-driven positions, these top SQL questions will sharpen your edge in interviews.

✅ Basics & Core Syntax

1. What is SQL?
SQL (Structured Query Language) is used to interact with relational databases — for querying, updating, and managing data.

2. Difference between WHERE and HAVING?

WHERE filters rows before aggregation
HAVING filters after aggregation

3. What are the different types of SQL statements?

DDL (Data Definition Language): CREATE, ALTER, DROP
DML (Data Manipulation Language): INSERT, UPDATE, DELETE
DQL: SELECT
DCL: GRANT, REVOKE
TCL: COMMIT, ROLLBACK, SAVEPOINT

4. What is the difference between INNER JOIN and LEFT JOIN?

INNER JOIN: Returns only matching records
LEFT JOIN: Returns all from left table + matching from right

5. What is a Primary Key vs Foreign Key?

Primary Key: Uniquely identifies each row
Foreign Key: References primary key in another table (used to maintain relationships)

✅ Queries & Clauses

6. How to find duplicate records in a table?

sqlCopyEditSELECT column_name, COUNT(*)
FROM table_name
GROUP BY column_name
HAVING COUNT(*) > 1;

7. What does GROUP BY do?
It aggregates rows that have the same values in specified columns.

8. How does DISTINCT work?
Removes duplicate records from a result set.

9. How to fetch the second highest salary from an Employee table?

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

10. Difference between DELETE, TRUNCATE, and DROP?

DELETE: Deletes rows, can be rolled back
TRUNCATE: Deletes all rows, can’t be rolled back
DROP: Removes table schema entirely

✅ Constraints, Indexes & Optimization

11. What are SQL Constraints?
Rules enforced on data: NOT NULL, UNIQUE, CHECK, DEFAULT, PRIMARY KEY, FOREIGN KEY

12. What is an Index?
A performance tuning feature — speeds up retrievals but slows down inserts/updates.

13. What is Normalization?
Organizing data to reduce redundancy:

1NF: Atomic values
2NF: No partial dependencies
3NF: No transitive dependencies

14. Difference between UNION and UNION ALL?

UNION: Removes duplicates
UNION ALL: Keeps all records (faster)

15. What is a View?
A virtual table created using a SQL query. It does not store data itself.

✅ Real-World & Advanced Scenarios

16. What are Stored Procedures?
A block of SQL code that performs a task, saved in the DB and reused.

17. What is a CTE (Common Table Expression)?
Temporary result set used inside a WITH clause for readable, reusable logic.

18. What are window functions in SQL?
Used to perform calculations across rows related to the current row, without collapsing the result set:
Examples: ROW_NUMBER(), RANK(), LEAD(), LAG()

19. Difference between OLTP and OLAP systems?

OLTP: Transactional systems (real-time)
OLAP: Analytical systems (data analysis/reporting)

20. What is ACID in databases?

Atomicity: All-or-nothing transactions
Consistency: Data stays valid
Isolation: Transactions don’t interfere
Durability: Changes are permanent

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Top 20 SQL Interview Questions for 2025 (With Answers)