Calculate churn rate per user
Last updated: July 27, 2025
Quick Overview
Write a query to compute churn rate grouped by date, handling edge cases like nulls and duplicates.
Robinhood
Data Manipulation (SQL/Python)
Data Scientist
Robinhood
July 27, 2025Data Scientist
Phone Screen
Data Manipulation (SQL/Python)
Hard
99
6
1,011 solved
Write a query to compute churn rate grouped by date, handling edge cases like nulls and duplicates.
This question from Robinhood'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
- 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
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
NULL handling and COALESCE
Common Table Expressions (CTEs)
Aggregate functions and GROUP BY
Index optimization and query performance
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 handle this if the data was spread across multiple databases?
- Can you rewrite this without using subqueries?
- How would you optimize this query for a table with 100 million rows?
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Practice SQL ProblemsSample Answer
Problem Understanding
To calculate the churn rate per user for Robinhood, we need to analyze user activity data over time. The churn rate is defined as the percentage of users who stop using the service over a specific per...
Approach
- Identify User Activity: Create a Common Table Expression (CTE) to extract distinct user activity data with the activity dates.
- Determine Churn: Use window functions to track the last ac...
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