SQL Bookstore Schema Practice | Dataford Interview Questions

Meta Logistics uses a bookstore payment dataset to monitor customer and catalog activity. Write a PostgreSQL query that summarizes valid completed book payments.

A payment counts only when status = 'paid' and amount > 0. Payments must reference an existing customer and book.

Requirements

Return the number of unique paying customers.
Return the number of unique books included in valid paid transactions.
Return the average payment amount per valid paid transaction, rounded to two decimal places.
Return one summary row with the aliases shown in the expected output.

Schema

customers

Column	Type	Description
customer_idPK	INT	Unique customer identifier
customer_name	VARCHAR(100)	Customer full name
email	VARCHAR(255)	Customer email address

books

Column	Type	Description
book_idPK	INT	Unique book identifier
title	VARCHAR(200)	Book title
author	VARCHAR(100)	Book author

payments

Column	Type	Description
payment_idPK	INT	Unique payment identifier
customer_id	INT	Customer associated with the payment
book_id	INT	Book associated with the payment
amount	DECIMAL(10,2)	Payment amount in dollars
status	VARCHAR(20)	Payment processing status

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