Calculate time to first action per user
Last updated: April 30, 2026
Quick Overview
Write a query to compute time to first action grouped by date, handling edge cases like nulls and duplicates.
Two Sigma
April 30, 20260
8
1,997 solved
Write a query to compute time to first action grouped by date, handling edge cases like nulls and duplicates.
This question from Two Sigma's Technical 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
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 would you do if this query needs to run every 5 minutes?
- How would you handle slowly changing dimensions in this scenario?
- Can you rewrite this without using subqueries?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task is to compute the time to first action for each user, grouped by date. The data involved includes user actions with timestamps and user identifiers. Each user may have multiple actions on a g...
Approach
- Identify the Data Structure: Start by understanding the schema of the actions table, which should include fields like
user_id,action_timestamp, and possiblyaction_type. - **Filter Rele...
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