In a database or other infoirnation system, a unique identifier for a data item or record is a combination of features of the data or a generated identifier that is not shared by any other item in the dataset. For example, in a geographic information system, we might use the physical location of a town as a identifier rather than name, as there are often many towns with the same name. Often these feature-based identifications are difficult to collect or maintain, or don't even exist, hence the frequent use of generated identifiers such as health or tax numers for people. Idetifiers may be unique in a single database (e.g. record number) or may be a global unique idientifer ({GUID}} such as URIs for the web.