Calculate DAU/MAU ratio per date
Last updated: June 8, 2026
Quick Overview
Write a query to compute DAU/MAU ratio grouped by date, handling edge cases like nulls and duplicates.
Cockroach Labs
June 8, 2026165
5
4,657 solved
Write a query to compute DAU/MAU ratio grouped by date, handling edge cases like nulls and duplicates.
Cockroach Labs asks this during the Onsite because data engineering skills are critical for the role. You should be comfortable with complex joins, window functions, CTEs, and performance optimization.
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?
- How would you handle this if the data was spread across multiple databases?
- How would you validate the correctness of your query results?
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Practice SQL ProblemsSample Answer
Problem Understanding
To compute the Daily Active Users (DAU) and Monthly Active Users (MAU) ratio per date, we need to aggregate user activity data over specific time frames. We assume we have a table named `user_activity...
Approach
- Calculate DAU: Use a DISTINCT count of
user_idperactivity_dateto get the number of unique users active on each date. - Calculate MAU: Use a window function to calculate the unique...
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