Comments by fatman45 (Top 4 by date)

COUNT(DISTINCT expression)

is giving you a count of unique values, but it doesn't make the whole subquery unique. So for instance if there are multiple duplicate SCAN_IDs (since it has no UNIQUE constraint) then you will get more results in your output than query 'a' would produce.

VPN is a secure tunnel over the internet to your private network. Using OpenVPN, you setup a VPN server on the private network. Using the server, you create a login, password, and self-signed SSL certificate for each remote user. The remote users install the OpenVPN client on their machine. When they want to connect, they run the client, enter their username and password, and the OpenVPN server issues them an IP address that is on your private network. The VPN "tunnel", as it is called, is secured by the SSL certificates. The OpenVPN server also "pushes", or forwards, the private network's DNS to the client. So they can access the database by it's regular name. By the way OpenVPN is already installed on some routers, so your private network may already have it. And if it doesn't have it, for many routers it is available as a package you can download and install on the router. You can read more about generic VPN here: https://en.wikipedia.org/wiki/Virtual_private_network and OpenVPN in the link I already gave you - just go to the downloads page, the main page is mostly talking about their cloud VPN for end users.