Queries with incomplete answers over semistructured data
Yaron Kanza, Werner Nutt, Yehoshua Sagiv · 1999
Semistructured data occur in situations where informationThe growing need to integrate data from heterogeneous lacks a homogeneous structure and is incomplete.Yet, up to sources and to access data sources with irregular or incomnow the incompleteness of information has not been reflected plete contents is the main motivation for research into semiby special features of query languages for semistructured structured data models and query languages for them.Semidata.Our goal is to investigate the principles of queries that structured data do not comply with a strict schema and allow for incomplete answers.We do not present, however, are inherently incomplete.Query languages for such data a concrete query language.should ,reflect these characteristics.Queries over classical structured data models contain a number of variables and conditions on these variables.An answer is a binding of the variables by elements of the database such that the conditions are satisfied.In the present paper, we loosen this concept in so far as we allow also answers that are partial, that is, not all variables in the query are bound by such an answer.Partial answers make it necessary to refine the model of query evaluation.The first modification relates to the satisfaction of conditions: under some circumstances we consider conditions involving unbound variables as satisfied.Second, in order to prevent a proliferation of answers, we only accept answers that are maximal in the sense that there are no assignments that bind more variables and satisfy the conditions of the query.