Li-min Liu, James Geller and Yehoshua Perl,

Abstract: In previous work, we have modeled a vocabulary given as a semantic network by an OODB (Object-Oriented Database). The OODB schema thus obtained provides a compact abstract view of the vocabulary. This enables fast traversal of the vocabulary by a user. In the semantic network vocabulary, the IS-A relationships express the specialization hierarchy. In our OODB modeling of the vocabulary, the SUBCLASS relationship expresses the specialization hierarchy of the classes and supports the inheritance of their properties. A typical IS-A path in the vocabulary has a corresponding shorter SUBCLASS path in the OODB schema. In the current paper we expose several cases where the SUBCLASS hierarchy fails to fully correspond to the IS-A hierarchy of the vocabulary. In these cases there exist traversal paths in the semantic network for which there are no corresponding traversal paths in the OODB schema. The reason for this failure is the existence of some IS-A relationships between concepts of two classes, that are not connected by a SUBCLASS relationship. This
phenomenon weakens the accuracy of our modeling. To rectify the situation we introduce a new OODB semantic relationship IS-A?to represent the existence of IS-A relationships between concepts of a pair of classes which are not connected via a SUBCLASS relationship. The resulting schema contains both SUBCLASS relationships and IS-A?relationships which completely models the ISA hierarchy of the vocabulary. We define a mixed class level traversal path to contain either SUBCLASS or IS-A? relationships. Consequently, each traversal path in the semantic network has a corresponding mixed traversal path in the OODB schema. Hence the introduction of the semantic OODB IS-A?relationship improves the modeling of semantic network vocabularies by OODBs.