Measure Retention After Product Launch | Dataford Interview Questions

Your question is Measure Retention After Product Launch. Start with the requirements and the three tables on the right.

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Problem

NovaPlay launched a major product update on 2024-04-01 and wants to understand whether user retention changed after the launch. Write a PostgreSQL query that uses a cohort table to compare retention for pre-launch and post-launch signup cohorts.

Requirements

Treat each user's cohort as the month of their signup_date from the cohort_table.
For each cohort month, calculate month 1 retention: the percentage of users who had at least one activity event in the calendar month immediately after their cohort month.
Label each cohort as pre_launch or post_launch based on whether the cohort month is before 2024-04-01.
Return one row per cohort month with cohort size, retained users, and retention rate rounded to 2 decimals.

Schema

users

Column	Type	Description
user_idPK	INT	Unique user identifier
signup_date	DATE	Date the user signed up
country	VARCHAR(50)	User country

cohort_table

Column	Type	Description
user_idPK	INT	User identifier
cohort_month	DATE	First day of the user's signup month

user_activity

Column	Type	Description
activity_idPK	INT	Unique activity event identifier
user_id	INT	User identifier tied to the activity
activity_date	DATE	Date of the activity event
activity_type	VARCHAR(50)	Type of activity event