This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Dual is a place holder, nothing more. It exists as a table containing one column called dummy with the value x. You use it to return constant values, such as the value of a sequence. The reason you use dual is it is guarenteed to only ever have one row and it is visible to all users.