Join sessions and rides to find churn rate
Last updated: September 9, 2025
Quick Overview
Write a query joining sessions and clicks to produce the combined churn rate.
Waymo
Data Manipulation (SQL/Python)
Data Scientist
Waymo
September 9, 2025Data Scientist
Take-home Project
Data Manipulation (SQL/Python)
Medium
51
3
2,437 solved
Write a query joining sessions and clicks to produce the combined churn rate.
This question from Waymo's Take-home Project 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
Data cleaning and transformation
Pandas vectorized operations and groupby
Date/time manipulation
Aggregate functions and GROUP BY
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
Subqueries and correlated subqueries
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 optimize this query for a table with 100 million rows?
- What indexes would you create to support this query?
- Can you rewrite this without using subqueries?
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Practice SQL ProblemsSample Answer
Problem Understanding
To calculate the churn rate for Waymo, we need to analyze two datasets: sessions and rides. The sessions table contains data on user sessions, including session IDs, user IDs, and timestamps. Th...
Approach
- Identify the user base: Start by determining the users who had at least one session in a given time frame from the
sessionstable. - Count completed rides: Join this user base with the...
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