geo_sales (sale_id, customer_id, region, country, amount) holds one row per sale. Customers can buy many times, in more than one country, and in more than one region.
Report the number of distinct customers who bought:
- •in each country (with its region);
- •in each region — a subtotal row with
country = 'ALL'; - •overall — one row with
region = 'ALL' and country = 'ALL'.
A customer counts once at every level they bought at, however many purchases or countries are involved.
Columns: region, country, customers. Sort by region alphabetically with the overall row last; within a region, countries alphabetically followed by the region's subtotal row.