Write a query to find DAU/MAU ratio from transactions

Last updated: April 12, 2026

Quick Overview

Write a SQL query to calculate DAU/MAU ratio from the transactions table, considering nulls and duplicates.

Dropbox

Data Manipulation (SQL/Python)

Data Scientist

Dropbox

April 12, 2026

Data Scientist

Technical Screen

Data Manipulation (SQL/Python)

Easy

3,173 solved

Write a SQL query to calculate DAU/MAU ratio from the transactions table, considering nulls and duplicates.

This question from Dropbox's Technical 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

Write syntactically correct SQL with proper JOIN and WHERE clauses
Use GROUP BY and aggregate functions appropriately
Handle NULL values correctly in your queries
Explain the query execution plan at a high level

Key Topics to Cover

Data cleaning and transformation

Aggregate functions and GROUP BY

Index optimization and query performance

Date/time manipulation

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

What would you do if this query needs to run every 5 minutes?
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 Problems

Sample Answer

Problem Understanding

To compute the DAU (Daily Active Users) to MAU (Monthly Active Users) ratio from the transactions table, we first need to identify the relevant data. The transactions table likely contains at least tw...

Approach

Select Unique Users: Use DISTINCT to select unique user_id values for DAU and MAU calculations.
Calculate DAU: Group by the date (transaction_date) to count unique users for each...

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