SQL Window Function Ranking | Dataford Interview Questions

Your question is SQL Window Function Ranking. Start with the requirements and the two tables on the right.

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Problem

Chargebee QA needs to validate invoice-ranking results in a billing report. Write a PostgreSQL query that ranks each active customer's paid invoices issued in June 2026 by invoice amount.

Requirements

Join customers to invoices and include only active customers.
Include invoices with a non-null paid_at value and an invoice_date from 2026-06-01 through 2026-06-30.
Return customer name, invoice number, invoice date, amount, and rank within that customer, highest amount first.
Use RANK() so invoices with equal amounts receive the same rank. Sort final output by customer name, rank, then invoice number.

Schema

customers

Column	Type	Description
customer_idPK	INTEGER	Unique Chargebee customer identifier.
customer_name	VARCHAR(100)	Display name of the customer.
is_active	BOOLEAN	Whether the customer account is active.

invoices

Column	Type	Description
invoice_idPK	INTEGER	Unique invoice identifier.
customer_id	INTEGER	Customer associated with the invoice.
invoice_number	VARCHAR(50)	Customer-facing Chargebee invoice number.
invoice_date	DATE	Date the invoice was issued.
amount	NUMERIC(12,2)	Invoice amount in the account currency.
paid_at	TIMESTAMP	Timestamp when the invoice was paid.