Window Functions for Top Customers | Dataford Interview Questions

Pacific Gas and Electric needs a regional view of customer electricity consumption for 2025. Write a PostgreSQL query that identifies the top three customers by total usage within each PG&E service region.

Requirements

Sum each customer's usage_kwh during calendar year 2025.
Include customers with no qualifying usage, treating their total as zero.
Rank customers independently within each region using a window function.
Return the top three customers per region, ordered by region, usage descending, and customer ID as a deterministic tie-breaker.

Schema

customers

Column	Type	Description
customer_idPK	INTEGER	Unique PG&E customer identifier
customer_name	VARCHAR(100)	Customer account name
region	VARCHAR(50)	PG&E service region
service_start_date	DATE	Date the customer's service began

usage_records

Column	Type	Description
usage_idPK	INTEGER	Unique usage record identifier
customer_id	INTEGER	Referenced customer identifier
usage_date	DATE	Date of the electricity usage measurement
usage_kwh	NUMERIC(12,2)	Measured electricity usage in kilowatt-hours

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