Join rides and users to find cumulative sum
Last updated: March 5, 2026
Quick Overview
Write a query joining rides and transactions to produce the combined cumulative sum.
Cloudflare
Data Manipulation (SQL/Python)
Data Scientist
Cloudflare
March 5, 2026Data Scientist
Onsite
Data Manipulation (SQL/Python)
Medium
6
7
2,279 solved
Write a query joining rides and transactions to produce the combined cumulative sum.
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)
Aggregate functions and GROUP BY
Date/time manipulation
JOIN types and when to use each
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
- What indexes would you create to support this query?
- How would you validate the correctness of your query results?
- How would you handle this if the data was spread across multiple databases?
- How would you handle slowly changing dimensions in this scenario?
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Practice SQL ProblemsSample Answer
Problem Understanding
We have two tables involved in this query: rides and transactions. The rides table contains information about individual rides, including ride ID, user ID, and a timestamp indicating when the ri...
Approach
- Join the Tables: Start by performing an INNER JOIN on the
ridesandtransactionstables using the user ID, as we want to match rides with their corresponding transactions. - **Select Rele...
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