Missing information (applicable and inapplicable) in relational databases

There has been some technical and justified criticism of the treatment of missing information in the data sublanguage SQL and in IBM's Database 2 system (a relational database management system). Some of this criticism has been directed (by mistake) at the relational model. The purpose of this paper is to clarify and extend the treatment of missing information by the relational model. The clarification places heavy emphasis on the semantic aspects of missing information. The extension, which is relatively minor, provides a systematic approach (independent of data type) to dealing with the inapplicability of certain properties to some objects. This extension does not invalidate any part of the present version of the relational model.