SQL Window Functions for Revenue | Dataford Interview Questions

Devoteam's analytics team needs a cohort-level view of user revenue for its data platform. Write a PostgreSQL query that aggregates completed transaction revenue per user, then calculates both revenue rank and cumulative revenue within each signup-month cohort.

Requirements

Derive each user's cohort from the month of signup_date.
Include users with no completed transactions, treating their revenue as zero.
Use window functions to calculate revenue_rank within each cohort and running_total ordered by revenue descending, with user_id as a deterministic tie-breaker.
Exclude users whose signup date is unknown and transactions that are not completed.

Schema

users

Column	Type	Description
user_idPK	INT	Unique user identifier
user_name	VARCHAR(100)	User display name
signup_date	DATE	Date the user signed up

transactions

Column	Type	Description
transaction_idPK	INT	Unique transaction identifier
user_id	INT	User associated with the transaction
amount	NUMERIC(10,2)	Revenue amount
status	VARCHAR(20)	Transaction processing status

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