You are given monthly performance data for delivery regions. Write a PostgreSQL query that compares each region’s current month performance to its previous month using a self-join. Return only months where a prior month exists, along with the month-over-month change in orders and revenue.
| Column | Type | Description |
|---|---|---|
| region_id | INT | Region identifier |
| region_name | VARCHAR(100) | Region name |
| month_start | DATE | First day of the month |
| orders_completed | INT | Completed orders in the month |
| gross_revenue | NUMERIC(12,2) | Gross revenue for the month |