Calculate DAU/MAU ratio per date

Last updated: September 23, 2025

Quick Overview

Write a query to compute DAU/MAU ratio grouped by date, handling edge cases like nulls and duplicates.

Scale AI

Data Manipulation (SQL/Python)

Data Scientist

Scale AI

September 23, 2025

Data Scientist

Onsite

Data Manipulation (SQL/Python)

Medium

133 solved

Write a query to compute DAU/MAU ratio grouped by date, handling edge cases like nulls and duplicates.

This question from Scale AI'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

Aggregate functions and GROUP BY

Index optimization and query performance

Common Table Expressions (CTEs)

Date/time manipulation

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?
How would you handle this if the data was spread across multiple databases?
What indexes would you create to support this query?
What would you do if this query needs to run every 5 minutes?

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Problem Understanding

The task is to compute the Daily Active Users (DAU) and Monthly Active Users (MAU) ratio per date. The data involved likely includes a user activity log with timestamps indicating when users were acti...

Approach

Identify the Data Structure: Assume we have a table named user_activity with columns user_id (identifier for the user) and activity_date (date of user activity).
Calculate DAU: U...

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