Calculate time to first action per user
Last updated: February 14, 2026
Quick Overview
Write a query to compute time to first action grouped by region, handling edge cases like nulls and duplicates.
Coinbase
February 14, 2026117
7
1,349 solved
Write a query to compute time to first action grouped by region, handling edge cases like nulls and duplicates.
This question from Coinbase's Onsite 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
- 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 this if the data was spread across multiple databases?
- How would you optimize this query for a table with 100 million rows?
- Can you rewrite this without using subqueries?
- How would you handle slowly changing dimensions in this scenario?
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