The VLDB Journal

, Volume 23, Issue 5, pp 747–769 | Cite as

Taxonomy-based relaxation of query answering in relational databases

  • Davide Martinenghi
  • Riccardo Torlone
Regular Paper


Traditional information search in which queries are posed against a known and rigid schema over a structured database is shifting toward a Web scenario in which exposed schemas are vague or absent and data come from heterogeneous sources. In this framework, query answering cannot be precise and needs to be relaxed, with the goal of matching user requests with accessible data. In this paper, we propose a logical model and a class of abstract query languages as a foundation for querying relational data sets with vague schemas. Our approach relies on the availability of taxonomies, that is, simple classifications of terms arranged in a hierarchical structure. The model is a natural extension of the relational model in which data domains are organized in hierarchies, according to different levels of generalization between terms. We first propose a conservative extension of the relational algebra for this model in which special operators allow the specification of relaxed queries over vaguely structured information. We also study equivalence and rewriting properties of the algebra that can be used for query optimization. We then illustrate a logic-based query language that can provide a basis for expressing relaxed queries in a declarative way. We finally investigate the expressive power of the proposed query languages and the independence of the taxonomy in this context.


Query languages Data model  Query relaxation Taxonomy Expressive power 



The authors acknowledge support from the EC’s FP7 “CUbRIK” project and from the Italian “GenData” PRIN project.


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Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.Politecnico di MilanoMilanItaly
  2. 2.Università Roma TreRomeItaly

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