Queries are posed in terms of a certain query language over the alphabet of the global ontology and are intended to extract a set of tuples of elements of the semantic domain. In accordance with what is typical in databases, each query is required to have an associated arity and that it extract only tuples of that arity. Given a source database for O, the tuples of interest are those that are guaranteed to be in the answer of the query for every model for O. with respect to the source database. In other words, certain answers are of interest. One of the most common ways to express knowledge on a domain of interest is to use class-based formalisms, in which knowledge is represented in terms of objects grouped into classes and relationships between classes. Examples are entity-relationship diagrams in databases, UML class diagrams in software engineering, and ontology languages...
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