Write a query to find churn rate from sessions
Last updated: September 17, 2025
Quick Overview
Write a SQL query to calculate churn rate from the sessions table, considering nulls and duplicates.
Mastercard
Data Manipulation (SQL/Python)
Data Scientist
Mastercard
September 17, 2025Data Scientist
Onsite
Data Manipulation (SQL/Python)
Medium
41
6
974 solved
Write a SQL query to calculate churn rate from the sessions table, considering nulls and duplicates.
This question from Mastercard'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
JOIN types and when to use each
Pandas vectorized operations and groupby
Aggregate functions and GROUP BY
NULL handling and COALESCE
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 would you do if this query needs to run every 5 minutes?
- How would you optimize this query for a table with 100 million rows?
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Practice SQL ProblemsSample Answer
Problem Understanding
To calculate the churn rate from the sessions table, we must first define what churn means in this context. Churn rate typically refers to the percentage of users who stop using a service over a speci...
Approach
- Identify Active Users: First, we need to identify all unique users who had sessions in the specified period.
- Determine Returning Users: Next, we will find users who had sessions in both ...
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