Online RPG

Hey everyone I have a great idea for a online RPG and I would like your help. But first to make one thing clear I am not a n00b I am pretty good with the code and have made several games already.

What I want to ask is.

A. Can I use SQL and is it a good idea
B. How do I make it multi player

Thats all I need to know for now any help in this will be :wink: (great)