Top SQL Interview Questions for Data Science in 2026

Why SQL Matters in Data Science Interviews

SQL remains the most tested skill in data science interviews. Whether you're applying to Google, Meta, or a fast-growing startup, you'll almost certainly face SQL questions. Here's what you need to know.

The Must-Know Topics

1. Joins

Understanding INNER, LEFT, RIGHT, and FULL OUTER joins is table stakes. But interviewers go deeper:

• Self-joins for hierarchical data
• Anti-joins using LEFT JOIN + WHERE IS NULL
• Cross joins for generating combinations

-- Classic: find employees who earn more than their manager
SELECT e.name, e.salary
FROM employees e
JOIN employees m ON e.manager_id = m.id
WHERE e.salary > m.salary;

2. Window Functions

Window functions separate strong candidates from average ones. Focus on:

• ROW_NUMBER(), RANK(), DENSE_RANK()
• LAG() and LEAD() for time-series comparisons
• Running totals with SUM() OVER (ORDER BY ...)

-- Find the top earner in each department
SELECT *
FROM (
    SELECT *, RANK() OVER (PARTITION BY dept ORDER BY salary DESC) as rk
    FROM employees
) ranked
WHERE rk = 1;

3. CTEs and Subqueries

Common Table Expressions make complex queries readable. Interviewers love multi-step problems that test your ability to break down logic.

WITH monthly_revenue AS (
    SELECT DATE_TRUNC('month', order_date) as month,
           SUM(amount) as revenue
    FROM orders
    GROUP BY 1
)
SELECT month,
       revenue,
       LAG(revenue) OVER (ORDER BY month) as prev_month,
       ROUND(100.0 * (revenue - LAG(revenue) OVER (ORDER BY month))
             / LAG(revenue) OVER (ORDER BY month), 1) as growth_pct
FROM monthly_revenue;

4. Aggregation and GROUP BY

• HAVING vs WHERE
• Counting distinct values
• Conditional aggregation with CASE WHEN

How to Prepare

1. Practice daily — consistency beats cramming
2. Write queries by hand — don't just read solutions
3. Time yourself — most interviews give 15-20 minutes per question
4. Explain your approach — communication matters as much as the answer

Practice These Questions

Ready to test your skills? Browse our SQL interview questions to practice with real problems from top companies.