Ntt { a Query Algebra for Pcte

In this paper we present NTT, a typed, set-oriented query algebra for PCTE. PCTE is an ISO standard of a DBMS whose data model is an extended ER model with attributed relationships and an object type hierarchy. The PCTE standard deenes navi-gational access to objects via an API (application program interface), but no set-oriented query language. NTT is the rst algebraic set-oriented query language for PCTE which fulllls the usual quality criteria for query languages, notably closure under the underlying data model and object preservation. The property of being closed is particularly important since a PCTE object base is mainly accessed by applications via an object-oriented API, and since closure is an indispensable condition for a view definition language. NTT is a lean, but expressive algebra and consists only of ve diierent operators; therefore, it is possible to formally deene the semantics of NTT.