Cohort Conversion Comparison | Dataford Interview Questions

Your question is Cohort Conversion Comparison. Start with the requirements and the two tables on the right.

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Problem

Business Context

Mirakl marketplace analytics needs to compare how effectively buyer cohorts convert after joining the marketplace.

Task

Write a PostgreSQL query that calculates and ranks buyer conversion rates by signup-month cohort for January through April 2025. A buyer is converted if they have at least one order with order_status = 'completed'.

Requirements

Derive each buyer's cohort from the month of signup_date using PostgreSQL date functions.
Include every eligible buyer in the cohort denominator, including buyers without orders or with only non-completed orders.
Return cohort size, converted buyers, conversion rate as a percentage rounded to two decimals, and the cohort rank using a window function.
Sort by conversion rate descending, then cohort month ascending.

Schema

marketplace_buyers

Column	Type	Description
buyer_idPK	INTEGER	Unique Mirakl marketplace buyer identifier
signup_date	DATE	Date the buyer joined the marketplace
acquisition_channel	VARCHAR(30)	Channel that generated the buyer signup

marketplace_orders

Column	Type	Description
order_idPK	INTEGER	Unique marketplace order identifier
buyer_id	INTEGER	Buyer associated with the order
order_date	DATE	Date the order was placed
order_status	VARCHAR(20)	Order lifecycle status