An SQL Extension Supporting User Viewpoints

In order to accomplish independence on the logical data organization, a relational DBMS must be capable of interpreting query language sentences which reference attributes belonging to different relations even if the necessary joins are not explicitly formulated (query inference problem). In this work we propose an SQL extension which supports concise formulation of queries through the dynamic definition of user viewpoints. User viewpoints are perspectives for accessing data; each user viewpoint defines a virtual derived relation (user viewpoint relation) which includes all those in the database scheme, each accessed by means of exactly one chain of system-activated equi-joins. Our approach to the definition of user viewpoint relations aims at reducing the global cost of query formulation, that is, the number of joins which must be explicitly written.