SQL Monthly Revenue by Region | Dataford Interview Questions

Write a SQL query to join orders, customers, and payments and calculate monthly revenue by region.

Use completed payments with a non-NULL payment timestamp. Treat a NULL customer region as Unknown and exclude payments without matching orders or customers.

Output

One row per revenue month and region, with columns revenue_month, region, and monthly_revenue.
Ignore rows with a NULL paid_at; order by revenue_month ascending, then region ascending.

Schema

customers

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

orders

Column	Type	Description
order_idPK	INT	Unique order identifier
customer_id	INT	Customer associated with the order
ordered_at	TIMESTAMP	Order creation timestamp

payments

Column	Type	Description
payment_idPK	INT	Unique payment identifier
order_id	INT	Order associated with the payment
paid_at	TIMESTAMP	Payment completion timestamp
amount	NUMERIC(12,2)	Payment amount
status	VARCHAR(40)	Payment processing status

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