Join sessions and users to find retention rate

Last updated: January 15, 2026

Quick Overview

Write a query joining sessions and sessions to produce the combined retention rate.

Lyft

Data Manipulation (SQL/Python)

Data Scientist

Lyft

January 15, 2026

Data Scientist

Technical Screen

Data Manipulation (SQL/Python)

Medium

443 solved

Write a query joining sessions and sessions to produce the combined retention rate.

Lyft 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

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

Window functions (ROW_NUMBER, RANK, LAG, LEAD)

Data cleaning and transformation

Index optimization and query performance

Subqueries and correlated subqueries

Aggregate functions and GROUP BY

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 indexes would you create to support this query?
How would you validate the correctness of your query results?
Can you rewrite this without using subqueries?
How would you handle this if the data was spread across multiple databases?

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Practice SQL Problems

Sample Answer

Problem Understanding

In this problem, we have two tables: sessions and users. The sessions table contains session information, including user IDs and timestamps, while the users table contains user details, includ...

Approach

Identify the First Session: Use a window function (like ROW_NUMBER()) to determine the first session for each user.
Join Tables: Join the users table with the sessions table on th...

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Lyft Data Scientist Interview Guide

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