Join messages and events to find retention rate
Last updated: January 16, 2026
Quick Overview
Write a query joining messages and sessions to produce the combined retention rate.
Neon
Data Manipulation (SQL/Python)
Data Scientist
Neon
January 16, 2026Data Scientist
Phone Screen
Data Manipulation (SQL/Python)
Medium
44
13
1,842 solved
Write a query joining messages and sessions to produce the combined retention rate.
This question from Neon's Phone Screen 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)
Index optimization and query performance
Date/time manipulation
Data cleaning and transformation
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 would you do if this query needs to run every 5 minutes?
- 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
In this problem, we are required to analyze user retention by joining two tables: messages and sessions. The messages table contains information about messages sent by users, including timestamp...
Approach
- Identify Key Metrics: Determine which columns are needed from both tables, such as user IDs, timestamps of messages, and session start times.
- Using CTEs: Create a CTE for each table to s...
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