THE that current database research can deal with advanced data applications trend is toward systems that go
beyond the standard “office” database application. This trend is reflected in the research on object-oriented databases .
Along with this trend, the need for better database end-user interfaces has been stressed [30],[36]. To this end, the 2-D
nature of a computer screen should be fully exploited. It seems natural that in order to achieve these goals, graphs are used as the basic data type. Graphs have indeed been an integral part of the database
design process ever since the introduction of semantic and, more recently, object-oriented data models [191, [20], [ 2 S ]
Their usage in data manipulation languages, however, is far less common. To deal with the language component, schemes in semantic and object-oriented data models are typically transformed into a conceptual data model such as the relational model. The required database language features then become those of the conceptual model.