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On the Semantics of Now in Databases
Curtis Dyreson, Assistant Prof., Washington State University
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  • James Clifford, Curtis E. Dyreson, Tomas Isakowitz, Christian S. Jensen, and Richard T. Snodgrass. "On the Semantics of Now in Databases." ACM Transactions on Database Systems 22(2), 1997, pp. 215--254. Abstract | PDF from ACM TODS | DBLP Electronic Edition
  • James Clifford, Curtis E. Dyreson, Richard T. Snodgrass, Tomas Isakowitz and Christian S. Jensen. Now. In The TSQL2 Temporal Query Language. Richard T. Snodgrass, editor. Kluwer Academic Press, 1995. pp. 383-392. DBLP Electronic Edition


While `now' is expressed in SQL as CURRENT_TIMESTAMP within queries, this value cannot be stored in the database. However, this notion of an ever-increasing current-time value has been reflected in some temporal data models by inclusion of database-resident variables, such as `now,' `until-changed,' `infinity,' `@' and `--.' Time variables are very desirable, but their use also leads to a new type of database, consisting of tuples with variables, termed a variable database.

This paper proposes a framework for defining the semantics of the variable databases of the relational and temporal relational data models. A framework is presented because several reasonable meanings may be given to databases that use some of the specific temporal variables that have appeared in the literature. Using the framework, the paper defines a useful semantics for such databases. Because situations occur where the existing time variables are inadequate, two new types of modeling entities that address these shortcomings, timestamps which we call now-relative and now-relative indeterminate, are introduced and defined within the framework. Moreover, the paper provides a foundation, using algebraic bind operators, for the querying of variable databases via existing query languages. This transition to variable databases presented here requires minimal change to the query processor. Finally, to underline the practical feasibility of variable databases, we show that database variables can be precisely specified and efficiently implemented in conventional query languages, such as SQL, and in temporal query languages, such as TSQL2.

Curtis E. Dyreson © 1993-2001. All rights reserved.
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