Encyclopedia of Database Systems

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

Possible Answers

  • Gösta GrahneEmail author
Reference work entry
DOI: https://doi.org/10.1007/978-1-4614-8265-9_1265


Consistent facts; Credulous reasoning; Maybe answer


In an incomplete database, which is a set of complete databases (possible worlds), the possible answer to a query is the set of tuples that are in the answer to the query, when posed on some possible world. The dual of the possible answer is the certain answer, which consists of all tuples true in the answer to the query when posed simultaneously on all possible worlds.

Key Points

For more information on the certain and possible answers, as well as on various (partial) orders on incomplete databases, see [1, 2].


Recommended Reading

  1. 1.
    Grahne G. The problem of incomplete information in relational databases. Berlin: Springer; 1991.zbMATHCrossRefGoogle Scholar
  2. 2.
    Libkin L. Aspects of partial information in databases. PhD Thesis, University of Pennsylvania. 1994.Google Scholar

Copyright information

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

Authors and Affiliations

  1. 1.Concordia UniversityMontrealCanada

Section editors and affiliations

  • Leonid Libkin
    • 1
  1. 1.School of InformaticsUniversity of EdinburghEdinburghUK