Join sessions and sessions to find time to first action
Last updated: April 8, 2026
Quick Overview
Write a query joining sessions and sessions to produce the combined time to first action.
Vercel
April 8, 202611
14
2,419 solved
Write a query joining sessions and sessions to produce the combined time to first action.
Data manipulation questions at Vercel test your ability to work with real-world datasets. This Phone Screen question evaluates your SQL proficiency, understanding of data modeling, and ability to derive insights from raw data.
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 handle this if the data was spread across multiple databases?
- What indexes would you create to support this query?
- How would you validate the correctness of your query results?
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Practice SQL ProblemsSample Answer
Problem Understanding
In this problem, we are dealing with two tables: sessions and actions. The sessions table contains details about user sessions, including session IDs and timestamps, while the actions table co...
Approach
- Identify Relevant Data: We need session IDs and their corresponding start times from the
sessionstable, and the timestamps of the first actions from theactionstable. - Join Tables: ...
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