Join sessions and events to find DAU/MAU ratio
Last updated: November 30, 2025
Quick Overview
Write a query joining sessions and users to produce the combined DAU/MAU ratio.
Reddit
Data Manipulation (SQL/Python)
Data Scientist
Data Scientist
Onsite
Data Manipulation (SQL/Python)
Medium
77
7
2,160 solved
Write a query joining sessions and users to produce the combined DAU/MAU ratio.
This question from Reddit'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
Data cleaning and transformation
Common Table Expressions (CTEs)
Pandas vectorized operations and groupby
NULL handling and COALESCE
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
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 validate the correctness of your query results?
- Can you rewrite this without using subqueries?
- How would you optimize this query for a table with 100 million rows?
- How would you handle slowly changing dimensions in this scenario?
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Practice SQL ProblemsSample Answer
Problem Understanding
To calculate the Daily Active Users (DAU) and Monthly Active Users (MAU) ratio for Reddit, we need to work with two datasets: the sessions table and the users table.
- sessions: This table r...
Approach
- Identify Date Range: Determine the date for which we want to calculate DAU and MAU.
- Aggregate DAU: Use a CTE to count distinct users from the
sessionstable for each day. - **Aggreg...
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