Top 30 SQL Interview Questions Asked in 2026 (With Answers)

SQL remains the single most-tested skill in data analyst, data engineer, and business analyst interviews across every major company in 2026 — from TCS and Infosys to Amazon and Google.

After analyzing thousands of interview experiences shared on Glassdoor, LinkedIn, and our own user reports, we've compiled the 30 most-asked SQL questions with clear, interview-ready answers you can use directly.

Pro Tip: Don't just memorize answers. Practice writing each query by hand. Interviewers test your thinking process, not just the final query. Use GhostMode's Code Playground to practice live.

📗 Basic SQL Questions (Fresher / 1-2 Years)

What is the difference between WHERE and HAVING clause?

Model Answer

WHERE filters rows before grouping — it cannot use aggregate functions. HAVING filters after GROUP BY and can use aggregate functions like COUNT(), SUM(), AVG().

Example: SELECT dept, COUNT(*) FROM emp GROUP BY dept HAVING COUNT(*) > 5

What are the different types of JOINs in SQL?

Model Answer

INNER JOIN — Returns matching rows from both tables.
LEFT JOIN — All rows from left + matching from right (NULL if no match).
RIGHT JOIN — All rows from right + matching from left.
FULL OUTER JOIN — All rows from both tables.
CROSS JOIN — Cartesian product of both tables.

What is the difference between DELETE, TRUNCATE and DROP?

Model Answer

DELETE: Removes specific rows. Can use WHERE. Can be rolled back. Logged operation.
TRUNCATE: Removes all rows. Cannot use WHERE. Cannot be rolled back. Faster.
DROP: Removes the entire table structure. Cannot be rolled back.

What is a PRIMARY KEY vs UNIQUE KEY?

Model Answer

PRIMARY KEY: Uniquely identifies each row. Cannot be NULL. Only one per table. Automatically creates a clustered index.
UNIQUE KEY: Also enforces uniqueness but can have NULL values (one NULL per column). Multiple unique keys allowed per table.

What is the difference between CHAR and VARCHAR?

Model Answer

CHAR(n): Fixed-length. Always stores n characters, padding with spaces if shorter. Faster for fixed-size data like country codes.
VARCHAR(n): Variable-length. Only stores actual characters used. More storage-efficient for varying data.

📘 Intermediate SQL Questions (2-5 Years)

Write a query to find the 2nd highest salary from an Employee table.

Model Answer — Multiple Approaches

-- Method 1: Using LIMIT/OFFSET
SELECT DISTINCT salary
FROM Employee
ORDER BY salary DESC
LIMIT 1 OFFSET 1;

-- Method 2: Using Subquery
SELECT MAX(salary) FROM Employee
WHERE salary < (SELECT MAX(salary) FROM Employee);

-- Method 3: DENSE_RANK() — Best for interviews
SELECT salary FROM (
  SELECT salary, DENSE_RANK() OVER (ORDER BY salary DESC) AS rnk
  FROM Employee
) t WHERE rnk = 2;

What are Window Functions? Name some commonly used ones.

Model Answer

Window functions perform calculations across a set of rows related to the current row, without collapsing them (unlike GROUP BY).

Common window functions:
• ROW_NUMBER() — Unique row number per partition
• RANK() — Rank with gaps for ties
• DENSE_RANK() — Rank without gaps
• LAG() / LEAD() — Access previous/next row value
• SUM() OVER / AVG() OVER — Running totals/averages

SELECT name, salary,
  DENSE_RANK() OVER (PARTITION BY dept ORDER BY salary DESC) as dept_rank
FROM Employee;

What is a CTE (Common Table Expression)? When do you use it?

Model Answer

A CTE is a named temporary result set defined using the WITH clause. It improves readability and can be referenced multiple times in the same query.

WITH SalesRanked AS (
  SELECT salesperson_id, SUM(amount) as total_sales,
    RANK() OVER (ORDER BY SUM(amount) DESC) as rnk
  FROM Sales
  GROUP BY salesperson_id
)
SELECT * FROM SalesRanked WHERE rnk <= 5;

Use CTEs when: the query is complex, you need to reference the same subquery multiple times, or for recursive queries.

Find duplicate records in a table.

Model Answer

-- Find duplicates
SELECT email, COUNT(*) as count
FROM Users
GROUP BY email
HAVING COUNT(*) > 1;

-- Delete duplicates, keep one
DELETE FROM Users
WHERE id NOT IN (
  SELECT MIN(id) FROM Users GROUP BY email
);

Q10

What is the difference between UNION and UNION ALL?

Model Answer

UNION: Combines result sets and removes duplicates. Slower due to deduplication step.
UNION ALL: Combines result sets and keeps all rows including duplicates. Faster. Use when you know there are no duplicates or duplicates are acceptable.

📕 Advanced SQL Questions (5+ Years / Senior Roles)

Q11

What is query optimization? How do you optimize a slow SQL query?

Model Answer

Key optimization techniques:

1. Use indexes on columns in WHERE, JOIN, ORDER BY
2. Avoid SELECT * — select only needed columns
3. Use EXISTS instead of IN for existence checks
4. Avoid functions on indexed columns in WHERE clause
5. Use EXPLAIN/EXPLAIN ANALYZE to understand query plan
6. Partition large tables for faster range scans
7. Avoid correlated subqueries in SELECT clause — use JOINs instead

Q12

Write a query to calculate running total of sales per day.

Model Answer

SELECT
  sale_date,
  daily_sales,
  SUM(daily_sales) OVER (
    ORDER BY sale_date
    ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
  ) AS running_total
FROM daily_sales_summary
ORDER BY sale_date;

🏢 Company-Specific SQL Questions

Amazon / Flipkart

Find customers who placed orders in every month of 2025.

Model Answer

SELECT customer_id
FROM Orders
WHERE YEAR(order_date) = 2025
GROUP BY customer_id
HAVING COUNT(DISTINCT MONTH(order_date)) = 12;

TCS / Infosys / Wipro

Find employees whose salary is greater than their department average.

Model Answer

SELECT e.name, e.department, e.salary
FROM Employee e
JOIN (
  SELECT department, AVG(salary) as avg_sal
  FROM Employee GROUP BY department
) dept_avg ON e.department = dept_avg.department
WHERE e.salary > dept_avg.avg_sal
ORDER BY e.department, e.salary DESC;

Google / Data Analyst Role

Calculate Month-over-Month (MoM) revenue growth.

Model Answer

SELECT
  month, revenue,
  LAG(revenue) OVER (ORDER BY month) as prev_month_revenue,
  ROUND(
    (revenue - LAG(revenue) OVER (ORDER BY month)) * 100.0 /
    NULLIF(LAG(revenue) OVER (ORDER BY month), 0), 2
  ) AS mom_growth_pct
FROM monthly_revenue
ORDER BY month;

✅ Last-Minute SQL Interview Tips

Always think out loud — explain your approach before writing the query
Ask about data size and edge cases (NULLs, duplicates) before finalizing
Know the difference between RANK vs DENSE_RANK vs ROW_NUMBER — commonly asked
Practice writing queries without references — muscle memory matters
For Data Analyst roles, expect business-context SQL, not just syntax
Know when to use CTE vs subquery vs temp table

🎙️

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Top 30 SQL Interview Questions Asked in 2026
(With Model Answers)

📗 Basic SQL Questions (Fresher / 1-2 Years)

📘 Intermediate SQL Questions (2-5 Years)

📕 Advanced SQL Questions (5+ Years / Senior Roles)

🏢 Company-Specific SQL Questions

✅ Last-Minute SQL Interview Tips

Practice These SQL Questions in a Live Mock Interview

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