Article on MySQL Number columns and UPDATE statements on numeric types

Here's something about mysql create table definitions that can easily catch you if you aren't careful. Consider this table definition:

CREATE TABLE screenshots (id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, votes INT);

What the user wanted was a simple table to keep track of user submitted screen shots. Of course the real table had a submission date, and name column but I've omitted those in order to focus on what can happen when you allow a numeric column to have NULL values.

http://www.gizmola.com/blog/archives/91-Mysql-Update-Null-+-1-is-Null!.html