A homogeneous relational model and query languages for temporal databases. (English) Zbl 0668.68118

A temporal database associates time values with data to indicate their periods of validity. Besides the current information also the old information resides in the database.
A temporal data model presented in the paper is a natural generalization of the relational data model (RDM), it is in a sense based on a temporal parametrization of static relations. The standard concepts of RDM are preserved.
A concept of temporal assignment is defined for an attribute A and a temporal domain v (a finite union of intervals) as a function from v into dom(A), the domain of A. An extension to temporal tuple and temporal relation are straight forward.
The data model is homogeneous: within a tuple a temporal domain does not vary from one attribute to another. A snapshot of a temporal relation is defined. The homogeneity requirement guarantees that the snapshots do not have nulls. The usual concepts of a key, anomalies, dependencies and normal forms are easily introduced through snapshots.
A relational algebra and a tuple relational calculus are introduced for the model and their equivalence is proven. Finally, other approaches to temporal databases and the problem how to relax the homogeneity requirement are discussed.
Reviewer: J.Šefránek


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