Calculate retention rate per region
Last updated: January 26, 2026
Quick Overview
Write a query to compute retention rate grouped by date, handling edge cases like nulls and duplicates.
Elastic
January 26, 202628
14
3,516 solved
Write a query to compute retention rate grouped by date, handling edge cases like nulls and duplicates.
Data manipulation questions at Elastic 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
- Write syntactically correct SQL with proper JOIN and WHERE clauses
- Use GROUP BY and aggregate functions appropriately
- Handle NULL values correctly in your queries
- Explain the query execution plan at a high level
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
- How would you handle slowly changing dimensions in this scenario?
- How would you handle this if the data was spread across multiple databases?
- 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
To calculate the retention rate per region, we need data that includes user activity, ideally with timestamps indicating when users first engaged with the product and when they returned. The key compo...
Approach
To build the query, I will follow these steps:
- Identify the data sources: We will need to join user activity data with user engagement data.
- Filter for first engagement: We will filter ...
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