Write a query to find cumulative sum from events
Last updated: March 9, 2026
Quick Overview
Write a SQL query to calculate cumulative sum from the events table, considering nulls and duplicates.
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Write a SQL query to calculate cumulative sum from the events table, considering nulls and duplicates.
Data manipulation questions at LinkedIn 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
- Solve complex analytical problems with elegant, readable SQL
- Optimize queries for large-scale datasets with partitioning and indexing
- Use recursive CTEs, lateral joins, and advanced window functions
- Design the data model alongside the query solution
- Discuss trade-offs between SQL and programmatic approaches (Python/pandas)
- Consider the operational aspects: query scheduling, incremental processing
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 validate the correctness of your query results?
- What would you do if this query needs to run every 5 minutes?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task is to compute a cumulative sum from an 'events' table, which contains records of various events (e.g., user actions, transactions) with timestamps. The query needs to handle NULL values grace...
Approach
- Identify Relevant Columns: Determine which columns are needed from the 'events' table, typically including 'event_time' (timestamp) and 'event_value' (the value to be summed).
- **Filter Out ...
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