An extension of the general entity manipulator language for global view definition

摘要

We extend the General Entity Manipulator (GEM) language with a facility for defining multidatabase views which we call global views. Our language permits global entity types to be defined with the full features of GEM entity types including generalization, entity-valued attributes, and set-valued attributes. The language supports the definition of entity mappings which define how global entity occurrences are materialized from local entity occurrences and attribute mappings which define conversions between local and global attributes. Entity level mappings are defined with a GEM data retrieval expression or the outer join operator. Attribute mappings are defined with a rich variety of conversion techniques such as string operators, database queries, and pre-compiled procedures. The language also accommodates the initial definition of a global view as well as the maintenance of an existing global view.