Join users and clicks to find DAU/MAU ratio
Last updated: October 24, 2025
Quick Overview
Write a query joining users and impressions to produce the combined DAU/MAU ratio.
Doordash
Data Manipulation (SQL/Python)
Data Scientist
Doordash
October 24, 2025Data Scientist
Technical Screen
Data Manipulation (SQL/Python)
Hard
18
5
3,180 solved
Write a query joining users and impressions to produce the combined DAU/MAU ratio.
Doordash asks this during the Technical Screen 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
- Solve complex analytical problems with elegant, readable SQL
- Optimize queries for large-scale datasets with partitioning and indexing
- Use recursive CTEs, lateral joins, and advanced window functions
- Design the data model alongside the query solution
- Discuss trade-offs between SQL and programmatic approaches (Python/pandas)
- Consider the operational aspects: query scheduling, incremental processing
Key Topics to Cover
Index optimization and query performance
Aggregate functions and GROUP BY
NULL handling and COALESCE
Pandas vectorized operations and groupby
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?
- 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 optimize this query for a table with 100 million rows?
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Practice SQL ProblemsSample Answer
Problem Understanding
In this task, we need to analyze user engagement for Doordash by calculating the Daily Active Users (DAU) to Monthly Active Users (MAU) ratio. The data involved includes:
- Users Table: Contains d...
Approach
To construct the query, we will follow these steps:
- Extract DAU: Count unique users per day in the impressions table.
- Extract MAU: Count unique users per month in the impressions table. ...
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