Calculate cumulative sum per product
Last updated: December 22, 2025
Quick Overview
Write a query to compute cumulative sum grouped by region, handling edge cases like nulls and duplicates.
OpenAI
December 22, 202539
6
4,268 solved
Write a query to compute cumulative sum grouped by region, handling edge cases like nulls and duplicates.
This question from OpenAI's Onsite tests practical data skills. The interviewer wants to see clean, efficient queries that handle edge cases like NULLs, duplicates, and large datasets.
What the Interviewer Expects
- Use advanced SQL features: window functions, CTEs, subqueries
- Write efficient queries that avoid common performance pitfalls
- Handle complex data transformations with multiple joins and aggregations
- Discuss indexing strategy and query optimization
- Address data quality issues: duplicates, missing values, outliers
Key Topics to Cover
How to Approach This
- Clarify the schema and expected output format before writing queries.
- Use CTEs (WITH clauses) to break complex queries into readable steps.
- Consider window functions (ROW_NUMBER, RANK, LAG, LEAD) for ranking and sequential analysis.
- Watch for NULLs, duplicates, and edge cases in JOINs and GROUP BY.
- For pandas, prefer vectorized operations over row-by-row iteration.
Possible Follow-up Questions
- Can you rewrite this without using subqueries?
- How would you handle slowly changing dimensions in this scenario?
- How would you optimize this query for a table with 100 million rows?
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Practice SQL ProblemsSample Answer
Problem Understanding
To compute the cumulative sum per product grouped by region, we will work with a dataset that likely includes fields such as product_id, region, and sales. The goal is to generate a cumulative t...
Approach
- Data Preparation: Start by cleaning the dataset to remove duplicates and handle NULL values in the
salescolumn. We will replace NULLs with 0 to ensure they do not negatively impact the cumul...
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