SQL for Membership Churn | Dataford Interview Questions

Your question is SQL for Membership Churn. Start with the requirements and the two tables on the right.

Run and submit as often as you like. When you're ready, talk me through your approach or go straight to the code.

Problem

Amazing Lash Studio wants a monthly view of membership churn based on activity in its reservation logs. Write a PostgreSQL query for January through April 2025.

A member counts in the monthly active base when they have at least one completed reservation during the month and their membership was active on the reservation date. A member counts as churned when their membership cancellation date falls in that same month.

Requirements

Return one row per month, including months with no qualifying reservations.
Show active members, churned members, and churn rate as a percentage rounded to two decimals.
Exclude cancelled, no-show, and NULL-status reservations.
Prevent division by zero and order results chronologically.

Schema

memberships

Column	Type	Description
membership_idPK	INTEGER	Unique membership identifier
member_id	INTEGER	Stable identifier for the member
member_name	VARCHAR(100)	Member name
start_date	DATE	Membership activation date
cancellation_date	DATE	Membership cancellation date

reservation_logs

Column	Type	Description
reservation_idPK	INTEGER	Unique reservation identifier
membership_id	INTEGER	Membership associated with the reservation
reservation_date	DATE	Date of the reservation
reservation_status	VARCHAR(20)	Reservation outcome