Calculate time to first action per user
Last updated: November 4, 2025
Quick Overview
Write a query to compute time to first action grouped by date, handling edge cases like nulls and duplicates.
Compass
November 4, 202529
4
143 solved
Write a query to compute time to first action grouped by date, handling edge cases like nulls and duplicates.
This question from Compass'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 validate the correctness of your query results?
- How would you handle slowly changing dimensions in this scenario?
- How would you optimize this query for a table with 100 million rows?
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Practice SQL ProblemsSample Answer
Problem Understanding
In this problem, we are tasked with calculating the time to the first action per user, grouped by date. The data we will be working with likely consists of a user actions table that includes at least ...
Approach
- Identify the Data Structure: Determine the table structure and the relevant columns. We will focus on the
user_idandaction_timestampcolumns. - Filter Data: Use a CTE to filter out n...
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