Write a query to find DAU/MAU ratio from sessions

Last updated: August 5, 2025

Quick Overview

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

Twitter/X

Data Manipulation (SQL/Python)

Data Scientist

Twitter/X

August 5, 2025

Data Scientist

Phone Screen

Data Manipulation (SQL/Python)

Medium

2,360 solved

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

This question from Twitter/X's Phone 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

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

Index optimization and query performance

Date/time manipulation

JOIN types and when to use each

Subqueries and correlated subqueries

Common Table Expressions (CTEs)

Data cleaning and transformation

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

Can you rewrite this without using subqueries?
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?

Sharpen Your Skills on Codemia

Practice similar problems with our interactive workspace, get AI feedback, and track your progress.

Practice SQL Problems

Sample Answer

Problem Understanding

The goal is to calculate the Daily Active Users (DAU) and Monthly Active Users (MAU) ratio from the sessions table.

Data Involved:

The sessions table includes columns such as user_id, `s...

Approach

To solve this problem, the following steps should be taken:

Calculate DAU: Use a COUNT(DISTINCT user_id) to find the number of unique users for a specific day.
Calculate MAU: Use a `CO...

Submit Your Answer

Markdown supported

Twitter/X Data Scientist Interview Guide

Interview process, tips, and preparation timeline