Encyclopedia of Database Systems

2018 Edition
| Editors: Ling Liu, M. Tamer Özsu

Now in Temporal Databases

  • Curtis E. Dyreson
  • Christian S. Jensen
  • Richard T. Snodgrass
Reference work entry
DOI: https://doi.org/10.1007/978-1-4614-8265-9_248

Synonyms

Current date; Current time; Current timestamp; Until changed

Definition

The word now is a noun in the English language that means “at the present time.” This notion appears in databases in three guises. The first use of now is as a function within queries, views, assertions, etc. For instance, in SQL, CURRENT_DATE within queries, etc., returns the current date as an SQL DATE value; CURRENT_TIME and CURRENT_TIMESTAMP are also available. These constructs are nullary functions.

In the context of a transaction that contains more than one occurrence of these functions, the issue of which time value(s) to return when these functions are invoked becomes important. When having these functions return the same (or consistent) value, it becomes a challenge to select this time and to synchronize it with the serialization time of the transaction containing the query.

The second use is as a database variable used extensively in temporal data model proposals, primarily as timestamp values...

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Recommended Reading

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  • Curtis E. Dyreson
    • 1
  • Christian S. Jensen
    • 2
  • Richard T. Snodgrass
    • 3
    • 4
  1. 1.Utah State UniversityLoganUSA
  2. 2.Department of Computer ScienceAalborg UniversityAalborgDenmark
  3. 3.Department of Computer ScienceUniversity of ArizonaTucsonUSA
  4. 4.Dataware VenturesTucsonUSA

Section editors and affiliations

  • Richard T. Snodgrass
    • 1
  • Christian S. Jensen
    • 2
  1. 1.University of ArizonaTucsonUSA
  2. 2.Aalborg UniversityAalborg ØstDenmark