Calculate time to first action per date
Last updated: March 30, 2026
Quick Overview
Write a query to compute time to first action grouped by user, handling edge cases like nulls and duplicates.
DE Shaw
March 30, 2026296
6
4,538 solved
Write a query to compute time to first action grouped by user, handling edge cases like nulls and duplicates.
DE Shaw asks this during the Take-home Project because data engineering skills are critical for the role. You should be comfortable with complex joins, window functions, CTEs, and performance optimization.
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
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?
- How would you handle this if the data was spread across multiple databases?
- How would you handle slowly changing dimensions in this scenario?
- 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 requires calculating the time to the first action taken by each user, grouped by date. The data involved typically includes user identifiers, action timestamps, and possibly other metadata. T...
Approach
- Identify the Relevant Tables: Determine which tables contain user actions and their timestamps. Assume we have a table
user_actionswith columns:user_id,action_timestamp, and `action_ty...
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