Sometimes isn't possible (or desirable) to find a Primary Key for a table. In this case the most common practice is to define a new artificial attribute (artificial in the sense it doesn't exist in the real world) so be the Primary Key. This attribute is called a Surrogate Key.
In general this Attribute has the Autonumber Property .
See Intelligent Versus Surrogate Keys for a good discussion on the subject.
Input Type and Suggest properties may help to solve the problem: "The biggest advantage for intelligent keys is that users understand what they mean whereas surrogate keys don't make any business sense..." which was mentioned at Intelligent Versus Surrogate Keys.
about "Custom applications can hide surrogate keys from the end user but ad-hoc database tools cannot: The user must learn what these columns are and how to use them to code joins.".