Attribute-Oriented View Definitions in Relational and Deductive Databases

In some database applications it is natural to deene individual attributes of a relation as derived attributes, separately from the deenition of other attributes. This approach minimizes the complexity of the schema. We propose a formalism in which rules are divided into two classes: \Deening rules" specify the existence of tuples in the relation, together with some of their attributes. \Reening rules" specify derived attributes of tuples that already exist according to the deening rules. A potential problem with this approach is that view deenitions may appear to be recursive, as some attributes of a relation are deened in terms of others. We show how this apparent recursion can be dealt with by identifying the attributes through which the recursion occurs. We present semantics based on these notions, and show how they allow the natural formulation of a motivating example. We discuss evaluation of rules with our semantics, demonstrating that goal-directed evaluation and incremental evaluation are possible, and that only rules deening derived attributes that are necessary for a query need to be evaluated in order to answer the query. Our proposal is directly relevant to the semantics of virtual columns in SQL3.