Closed World Databases Opened Through Null Values
Georg Gottlob, Roberto V. Zicari · 1988
We propose a new approach to the treatment of null-valued 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 speci-fication 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 ac-cordingly. This paper describes ongoing work. We state some open problems to be solved iu order to render our approach more operational 1.