Not sure you can use straight sql to produce the results you want from the current structure. From the examples in the manual , it looks like the suggested approach is to find the country when the user first hits your site. The issue lies around the fact that the IPs are a range in the country table so each IP in the visitors table needs a query to match the range data
My suggestion is to add an additional column to the visitors table and figure out the country when the IP is first detected. Then create an index on this column and use a simple SQL query to get the results
$sql = "select count(distinct(country)) as visits, country from visitors";
You could do this with a PHP / SQL combination and create a list of the visitors IPs, incrementing it each time thru the loop until all the records are done...this is slower and more CPU more intensive.
PS: If you're not Dutch, you're not much
