Hideko S. Kunii 
Graph Data Model [PDF ebook] 
and Its Data Language

Complex databases can be understood well with visual representation. A graph is a very intuitive and rational structure to visually represent such databases. Graph Data Model (GDM) proposed by the author formalizes data representation and operations on the data in terms of the graph concept. The GDM is an extension of the relational model toward structural representation. In this model, a database is defined by a schema graph where nodes represent record types and arcs represent link types that are relationships between two record types. The capabilities of the GDM include direct representation of many-to-many relationships and of the relationships within a single record type. The characteristic operators are those associated with links: existential, universal, numerical and transitive link operators. Graph Data Language (GDL) is a data language based on this GDM. The essence of the GDL is path expressions used for formulation of queries. The concepts of GDM and GDL have actually been implemented by Ricoh Co., Ltd. and a system based on these concepts is commercially available for many UNIX machines.