10 Most Asked SQL Interview Questions with Answers 2025

Q1: Difference between WHERE and HAVING?

WHERE filters individual rows before aggregation — cannot use aggregate functions. HAVING filters groups after aggregation — can use COUNT, SUM, AVG.

-- CORRECT
SELECT dept, COUNT(*) AS cnt FROM employees GROUP BY dept HAVING cnt > 5;

Q2: How to check for NULL?

NULL is not equal to NULL. Always use IS NULL or IS NOT NULL. COALESCE(col, default) replaces NULL with a value.

Q3: UNION vs UNION ALL?

UNION removes duplicates (sorts result — slower). UNION ALL keeps all rows including duplicates (faster). Use UNION ALL unless you need deduplication.

Q4: Find second highest salary.

SELECT MAX(salary) FROM employees WHERE salary < (SELECT MAX(salary) FROM employees);
-- Or:
SELECT DISTINCT salary FROM employees ORDER BY salary DESC LIMIT 1 OFFSET 1;

Q5: Find duplicate emails.

SELECT email, COUNT(*) FROM users GROUP BY email HAVING COUNT(*) > 1;

Q6: Delete duplicates keeping one.

DELETE FROM users WHERE id NOT IN (SELECT MIN(id) FROM users GROUP BY email);

Q7: Employees earning above department average.

SELECT e.name, e.salary FROM employees e
JOIN (SELECT dept_id, AVG(salary) AS avg FROM employees GROUP BY dept_id) d
  ON d.dept_id = e.dept_id WHERE e.salary > d.avg;

Q8: What is a self join?

A table joining itself. Classic use: employee-manager hierarchy where both are in the same table.

Q9: What is a stored procedure?

Precompiled SQL code stored in the database, called with CALL. Can have IN/OUT parameters, control flow (IF/LOOP), and transactions. Reduces network round-trips.

Q10: What is ACID?

Atomicity (all or nothing), Consistency (valid state), Isolation (concurrent safety), Durability (survives crashes). These are the properties guaranteed by database transactions.