Is it possible to decrement a variable in MySQL?

Question

I\'m wanting to decrement a variable in a MySQL table by one everytime an UPDATE query is ran.

What I have is this, which isn\'t working:

UPDATE forum

Answer 1

UPDATE forum SET replys = reply - 1 WHERE fid = '$id'

Answer 2

of course:

UPDATE forum SET replies=replies-1 WHERE fid = ?