PIQUE: A relational query language without relations

摘要

Relational database systems have gone far towards providing users with physical data independence. To use a relational database, users need not know the physical storage structures of relations, and are protected from changes in these structures. However, a user must still navigate among relations. In other words, if the information needed to answer a question spans several relations, he must explicitly specify how these relations are to be combined.Physical data independence is not enough. A user should also be afforded some degree of structural data independence. More specifically, he should be able to pose queries without having to explicitly navigate among relations in the database. Instead, the system should do the navigation for him.We consider universal scheme interfaces as a means for automatic database navigation, and introduce the concept of a generator as central to such navigation. We describe a particular generator based on the semantic notions of reducible and irreducible facts, and present PIQUE, an attribute-based query language designed to work with this generator.PIQUE is a concise, yet powerful, language with natural semantics. A distinguishing feature of PIQUE is that tuple variables in queries are bound implicitly and that the logical connectives “and”, “or,” and “not” can affect the binding, and, therefore, take on “semantic overtones.” Furthermore, the semantic interpretation PIQUE gives to these connectives is more natural than the one given by most other query languages.In the Appendix, we present PIQUE's formal syntax and semantics, along with the proof that PIQUE is relationally complete.