Write a query to find retention rate from users
Last updated: June 13, 2026
Quick Overview
Write a SQL query to calculate retention rate from the users table, considering nulls and duplicates.
Mastercard
Data Manipulation (SQL/Python)
Data Scientist
Mastercard
June 13, 2026Data Scientist
Onsite
Data Manipulation (SQL/Python)
Medium
9
5
1,539 solved
Write a SQL query to calculate retention rate from the users 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
Common Table Expressions (CTEs)
Subqueries and correlated subqueries
Index optimization and query performance
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
- What would you do if this query needs to run every 5 minutes?
- 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
To calculate the retention rate from the users table, we need to analyze user activity over different time frames. The primary data involved includes user IDs, their registration dates, and their acti...
Approach
- Identify Active Users: First, we will identify users who have been active in a given time frame (e.g., month) after their registration.
- Calculate Total Users: Next, we will calculate t...
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