Write a query to find churn rate from messages
Last updated: July 18, 2025
Quick Overview
Write a SQL query to calculate churn rate from the messages table, considering nulls and duplicates.
Tesla
July 18, 20257
5
2,500 solved
Write a SQL query to calculate churn rate from the messages table, considering nulls and duplicates.
Data manipulation questions at Tesla test your ability to work with real-world datasets. This Onsite 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
- What would you do if this query needs to run every 5 minutes?
- Can you rewrite this without using subqueries?
- How would you handle slowly changing dimensions in this scenario?
- How would you validate the correctness of your query results?
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Practice SQL ProblemsSample Answer
Problem Understanding
To calculate the churn rate from the messages table, we need to understand the data involved. The messages table likely contains columns such as user_id, message_id, message_date, and `statu...
Approach
- Data Cleaning: Start by filtering out NULL values in the
user_idcolumn to ensure we only consider valid users. 2. Identifying Active Users: Determine the set of unique users who have sen...
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