Join events and messages to find cumulative sum
Last updated: November 17, 2025
Quick Overview
Write a query joining events and clicks to produce the combined cumulative sum.
Discord
Data Manipulation (SQL/Python)
Data Scientist
Discord
November 17, 2025Data Scientist
Phone Screen
Data Manipulation (SQL/Python)
Medium
75
14
4,288 solved
Write a query joining events and clicks to produce the combined cumulative sum.
This question from Discord's Phone Screen 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
Data cleaning and transformation
NULL handling and COALESCE
Date/time manipulation
Subqueries and correlated subqueries
Aggregate functions and GROUP BY
Index optimization and query performance
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 validate the correctness of your query results?
- Can you rewrite this without using subqueries?
- How would you handle slowly changing dimensions in this scenario?
- What indexes would you create to support this query?
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Practice SQL ProblemsSample Answer
Problem Understanding
In this problem, we have two tables: events and messages. The events table contains records of user interactions (like joins) with timestamps, while the messages table contains user messages a...
Approach
- Join Tables: We will perform an INNER JOIN between the
eventsandmessagestables on a suitable key (e.g., user ID) and ensure that timestamps from both tables are considered. - **Cumulat...
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