Write a query to find cumulative sum from events
Last updated: January 9, 2026
Quick Overview
Write a SQL query to calculate cumulative sum from the events table, considering nulls and duplicates.
NVIDIA
January 9, 20268
8
4,764 solved
Write a SQL query to calculate cumulative sum from the events table, considering nulls and duplicates.
Data manipulation questions at NVIDIA test your ability to work with real-world datasets. This Take-home Project question evaluates your SQL proficiency, understanding of data modeling, and ability to derive insights from raw data.
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
- What indexes would you create to support this query?
- How would you optimize this query for a table with 100 million rows?
- How would you validate the correctness of your query results?
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Practice SQL ProblemsSample Answer
Problem Understanding
The events table contains records of various events, likely with columns such as event_id, event_value, and event_date. The goal is to calculate the cumulative sum of event_value over time, ...
Approach
- Identify Key Columns: Focus on
event_valuefor the cumulative sum andevent_datefor ordering. - Handle NULL Values: Use COALESCE to treat NULL event values as zero.
- **Use Window Fu...
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