Write a query to find time to first action from events
Last updated: October 1, 2025
Quick Overview
Write a SQL query to calculate time to first action from the events table, considering nulls and duplicates.
Neon
October 1, 20251
3
1,482 solved
Write a SQL query to calculate time to first action from the events table, considering nulls and duplicates.
This question from Neon'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
- 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
- Can you rewrite this without using subqueries?
- How would you validate the correctness of your query results?
- What indexes would you create to support this query?
- How would you handle slowly changing dimensions in this scenario?
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Practice SQL ProblemsSample Answer
Problem Understanding
The events table contains records of user actions with various timestamps. Our goal is to calculate the time taken for each user to perform their first action after they were registered. This involves...
Approach
- Identify Relevant Data: We need user registration dates and action timestamps from the events table.
- CTE for First Action: Create a Common Table Expression (CTE) to retrieve the first a...
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