Closed World Databases Opened Through Null Values

We propose a new approach to the treatment of nullvalued attributes in the relational model. The approach is based on the new concept of locally-controlled open world database. A locally-controlled open world dab base permits the definition of portions of a traditional closed world database as open-world. Attributes, part of a relation, or entire relations can be explicitly defined as ‘open” through the insertion of null-values. Under this assumption, we consider three different types of null values: the standard unknown, does not e&t, and the new one, called open. We give a complete formal specification of the semantics of these null values. We extend the arithmetic and logical operators to cover nulls and outline how relational operators cau be extended accordingly. This paper describes ongoing work. We state some open problems to be solved iu order to render our approach more operational