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Capital One·Data Scientist·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Capital One data scientist interview, SQL round. One question, but it had a lot of moving parts and the constraints made it trickier than it looked on the surface.

Questions Asked (1)

Q1

Given a schema with customers, orders, order_items, and products tables, write a single SQL statement that returns the product category with the highest total delivered revenue in August 2025 for each region. Revenue is qty times unit_price, filtered to delivered orders only. Output region, category, and total_revenue. Break ties by picking the lexicographically smallest category. You must use exactly one subquery or one CTE, and no window functions.

Data ModelingProduct Analytics & Metrics
Author's notes

The no-window-functions constraint is what got me.

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AI HintsAI Generated

Suggested Approach

First, use a single CTE to compute total delivered revenue per region, category, and month (filtering to August 2025 and delivered orders). Then, in the main query, join that CTE to a subquery that finds the maximum revenue per region, and use a tie-breaker (lexicographically smallest category) to select the top category per region.

Pro tip: When breaking ties, use a window function alternative like ROW_NUMBER() with ORDER BY revenue DESC, category ASC—but since window functions are disallowed, you can achieve the same with a correlated subquery or a self-join on the aggregated CTE, ensuring deterministic results.

1. Understand the schema and requirements

Identify the relevant tables (customers, orders, order_items, products) and the join keys. Clarify that revenue = qty * unit_price, only delivered orders, and filter to August 2025.

2. Aggregate revenue by region and category

Write a CTE that joins orders, order_items, and products, filters for delivered orders in August 2025, and computes SUM(qty * unit_price) grouped by region and category.

3. Find the maximum revenue per region

In the main query, use a subquery to get the max total_revenue for each region from the CTE, then join back to the CTE to get the category(ies) that achieve that max.

4. Apply tie-breaking and select final output

If multiple categories tie for max revenue in a region, pick the lexicographically smallest category. Use ORDER BY region, category and a LIMIT 1 per region (e.g., via a correlated subquery or DISTINCT ON in some dialects) to return one row per region.

Key Points to Mention

  • Correctly joining orders, order_items, and products to compute revenue.
  • Filtering for delivered orders only (e.g., order status = 'delivered') and August 2025 (e.g., order_date BETWEEN '2025-08-01' AND '2025-08-31').
  • Using a single CTE to pre-aggregate revenue by region and category, which simplifies the main query.
  • Handling ties by ordering categories lexicographically and selecting the first one.
  • Ensuring the final output includes region, category, and total_revenue, with one row per region.
  • Avoiding window functions and using only one subquery or CTE as required.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.