An extension to SQL, called the SQL extended normal form (XNF), is discussed. It enhances relational technology by a composite object facility, which comprises not only extraction of composite objects from existing databases but also efficient navigation and manipulation facilities provided by an appropriate application programming interface. The language itself allows sharing of the database among normal form SQL applications and composite object applications. It provides proper subsetting of the database and subsequent structuring, exploiting subobject sharing and recursion, all based on its powerful composite object constructor concept, which is closed under the language operations. XNF is integrated into the relational framework, thus benefiting from the available technology such as relational engine and query optimization.>

Recently, extensions for relational database management systems (DBMS) have been proposed to support also hierarchical structures (complex objects). These extensions have been mainly implemented on top of an existing DBMS. Such an approach leads to many disadvantages not only from the conceptual point of view but also from performance aspects. Thus paper reports on a 3-year effort to design and prototype a DBMS to support a generalized relational data model, called extended NF2 (Non First Normal Form) data model which treats flat relations, lists, and hierarchical structures in a uniform way. The logical data model, a language for this model, and alternatives for storage structures to implement generalized relations are presented and discussed.