For each country and each month in which it had orders, return country, month (YYYY-MM), revenue, prev_revenue (revenue in that country's previous month with orders, or NULL) and pct_change (rounded to 1 decimal place, NULL for a country's first month). Sort by country, then month.
Schema
customers(id, name, country, signup_date)
orders(id, customer_id, order_date, total)