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|Title: ||On the Semantics of "Now" in Databases|
|Authors: ||Clifford, James|
Jensen, Christian S.
Snodgrass, Richard T.
|Issue Date: ||Jun-1996|
|Publisher: ||Stern School of Business, New York University|
|Series/Report no.: ||IS-96-03|
|Abstract: ||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," "â," "@" 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
This paper proposes a framework for defining the semantics of the variable databases of 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
|Appears in Collections:||IOMS: Information Systems Working Papers|
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