Calculate cumulative sum per region
Last updated: March 11, 2026
Quick Overview
Write a query to compute cumulative sum grouped by user, handling edge cases like nulls and duplicates.
Cloudflare
Data Manipulation (SQL/Python)
Data Scientist
Cloudflare
March 11, 2026Data Scientist
Onsite
Data Manipulation (SQL/Python)
Medium
0
5
466 solved
Write a query to compute cumulative sum grouped by user, handling edge cases like nulls and duplicates.
This question from Cloudflare'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
Common Table Expressions (CTEs)
JOIN types and when to use each
Data cleaning and transformation
Index optimization and query performance
NULL handling and COALESCE
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
- How would you handle slowly changing dimensions in this scenario?
- How would you validate the correctness of your query results?
- How would you handle this if the data was spread across multiple databases?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task is to compute the cumulative sum of a specified metric (e.g., user spending) grouped by user and region. The relevant data is likely stored in a table, say user_transactions, which contains...
Approach
- Identify the Data: Start by confirming the structure of the
user_transactionstable, ensuring it has the necessary columns. - Filter Nulls: Use
COALESCEto treat NULL `transaction_amou...
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