Rolling Retention Rates by Cohort | Dataford Interview Questions

Your question is Rolling Retention Rates by Cohort. Start with the requirements and the two tables on the right.

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Problem

The OpenAI ChatGPT analytics team wants to compare engagement across user signup cohorts. Write a PostgreSQL query that measures activity during each user's first ninety days after signup.

Requirements

Group users by their signup_date cohort.
Calculate daily retention as the number of distinct users active on each cohort date divided by the cohort size.
Calculate the ninety-day rolling average of daily retention for each cohort using a window function ordered by activity date.
Exclude activity before signup, activity after day 90, users without a signup date, and rows with a null activity date. Return results ordered by cohort and activity date.

Schema

users

Column	Type	Description
user_idPK	INTEGER	Unique user identifier
signup_date	DATE	Date the user signed up

user_activity

Column	Type	Description
activity_idPK	INTEGER	Unique activity record
user_id	INTEGER	User associated with the activity
activity_date	DATE	Date of the activity