Geonames database query locally

If I import the Geonames database to a MySQL or SQLite database, how can query the database by latitude and longitude? I need to find the current country code based on a given geolocation (latitude and longitude). In other words, if I have a database with gps coordinates, how can I accurately get the closest one to a given latitude and longitude?

I'm looking for a standalone solution because of rate limits of most reverse geocoding services.

Thank you.