Write a query to find churn rate from clicks
Last updated: May 21, 2026
Quick Overview
Write a SQL query to calculate churn rate from the clicks table, considering nulls and duplicates.
Visa
Data Manipulation (SQL/Python)
Data Scientist
Visa
May 21, 2026Data Scientist
Take-home Project
Data Manipulation (SQL/Python)
Medium
1
7
3,262 solved
Write a SQL query to calculate churn rate from the clicks table, considering nulls and duplicates.
This question from Visa'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
JOIN types and when to use each
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
- How would you handle slowly changing dimensions in this scenario?
- Can you rewrite this without using subqueries?
- How would you handle this if the data was spread across multiple databases?
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Practice SQL ProblemsSample Answer
Problem Understanding
To calculate the churn rate from the clicks table, we need to understand the data involved. The clicks table likely contains records of user clicks, where each record may include a user ID, click time...
Approach
- Identify the time frame: Determine the period over which we want to calculate churn (e.g., the last month).
- Select Unique Users: Create a Common Table Expression (CTE) to extract unique ...
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