MySQL: Increment an exisitng value
Contributed by qmchenry 
February 24, 2007 
Tagged: MySQL
This slick MySQL syntax allows you to increment or decrement an existing number in a table without first having to read the value. This is a nice way to increment an access counter.
To increment the value ‘counter’ by one for the row in table ‘images’ where ‘image_id’ is ‘15′, use:
UPDATE images SET counter=counter+1 WHERE image_id=15
To decrement the value, use ‘counter=counter-1′ instead. Incrementing or decrementing by other values (or using whatever valid arithmetic arm flexing you need) will work, too.