Comparing two fields

http://blog.sforce.com/sforce/2011/03/a-neat-soql-trick-.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+SforceBlog+%28Force.com+Blog%29

Here's a cool trick where you can compare two fields within the query using a formula field.

There is some concern that it causes more queries in the back end.