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.