Help With Ajax I cant figure it out...

[JDameron91 0]

Hello, I have recentlty ran into a problem on my website: When a user logs in, it adds him to an active users list in the MySQL database, but if they don't click the "logout" button, and just leave the site, it keeps them in the active users list forever until I manually go into the database and take them out. I heard from a friend that you could use AJAX to find out if the user is active, if he's not and he stays inactive for a certain amount of time, then it will take him out of the active users list. So I go to learn AJAX because all of the other languages I know are easy, this was not so with AJAX; I can't even make a working request to the database!

So what I need is a way for the user that exits the site without clicking the "logout" button to be deleted from the active users list. I would be very happy if you gave me a code, tutorial or even refer me to a helpful site about AJAX.
All and any help is greatly appreciated. =]

Edited March 23, 2007 by JDameron91 (see edit history)