Ranking Customers with Window Functions | Dataford Interview Questions

Your question is Ranking Customers with Window Functions. 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

Entergy analysts need to compare customer energy consumption across service regions and billing months. Write a PostgreSQL query using the customers and energy_usage tables to rank customers by their total monthly usage within each Entergy region.

Requirements

Restrict usage records to January and February 2025.
Aggregate multiple usage readings for the same customer and month.
Use RANK() partitioned by region and month, with the highest usage receiving rank 1. Preserve ties with the same rank.
Return region, month, customer, total usage, and rank, ordered by month, region, rank, and customer ID.

Schema

customers

Column	Type	Description
customer_idPK	INT	Unique customer identifier
customer_name	VARCHAR(100)	Customer display name
region	VARCHAR(50)	Entergy service region

energy_usage

Column	Type	Description
usage_idPK	INT	Unique usage reading identifier
customer_id	INT	Customer identifier associated with the reading
usage_date	DATE	Date on which energy usage was recorded
kwh_used	NUMERIC(12,2)	Energy consumed in kilowatt-hours