letting members rate comments

i have a message area on my website and was wondering how you would go about letting members rate other members comments...I presume its a matter of having a form with a check box with hot or not....and that form passes to an action page, which adds +1 to the hot or not column in the message table depending on what has been checked in the check box. The action page would obviously prevent people from rating themselves, and also prevent others from rating a comment multiple times.
Anyway, was just wondering if anyone knew of the code which would add +1 to a number in a column....cheers