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On Preservation under Homomorphisms and Unions of Conjunctive Queries
, 2006
"... Unions of conjunctive queries, also known as selectprojectjoinunion queries, are the most frequently asked queries in relational database systems. These queries are definable by existential positive firstorder formulas and are preserved under homomorphisms. A classical result of mathematical log ..."
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Cited by 25 (4 self)
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Unions of conjunctive queries, also known as selectprojectjoinunion queries, are the most frequently asked queries in relational database systems. These queries are definable by existential positive firstorder formulas and are preserved under homomorphisms. A classical result of mathematical
Homomorphism Preservation Theorems
, 2008
"... The homomorphism preservation theorem (h.p.t.), a result in classical model theory, states that a firstorder formula is preserved under homomorphisms on all structures (finite and infinite) if and only if it is equivalent to an existentialpositive formula. Answering a longstanding question in fin ..."
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Cited by 27 (0 self)
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theory via a correspondence between existentialpositive formulas and unions of conjunctive queries. A further result of this article strengthens the classical h.p.t.: we show that a firstorder formula is preserved under homomorphisms on all structures if and only if it is equivalent to an existential
Verification of ConjunctiveQUery Based . . .
, 2011
"... We introduce semantic artifacts, which are a mechanism that provides both a semantically rich representation of the information on the domain of interest in terms of an ontology, including the underlying data, and a set of actions to change such information over time. In this paper, the ontology is ..."
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is specified as a DLLite TBox together with an ABox that may contain both (known) constants and unknown individuals (labeled nulls, represented as Skolem terms). Actions are specified as sets of conditional effects, where conditions are based on conjunctive queries over the ontology (TBox and ABox
Scalable Containment for Unions of Conjunctive Queries under Constraints
, 2013
"... We consider the problem of query containment under ontological constraints, such as those of RDFS. Query containment, i.e., deciding whether the answers of a given query are always contained in the answers of another query, is an important problem to areas such as database theory and knowledge repre ..."
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representation, with applications to data integration, query optimization and minimization. We consider unions of conjunctive queries, which constitute the core of structured query languages, such as SPARQL and SQL. We also consider ontological constraints or axioms, expressed in the language of Tuple
Homomorphisms Preserving Trellis Languages
 JOURNAL OF AUTOMATA, LANGUAGES AND COMBINATORICS
"... It is proved that a homomorphism h preserves the class of languages recognized by trellis automata augmented with the capacity of handling the empty string (or, equivalently, generated by linear conjunctive grammars) if and only if either h is injective, or h maps every symbol to the empty string. ..."
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It is proved that a homomorphism h preserves the class of languages recognized by trellis automata augmented with the capacity of handling the empty string (or, equivalently, generated by linear conjunctive grammars) if and only if either h is injective, or h maps every symbol to the empty string. The
Local Transformations and ConjunctiveQuery Equivalence
"... Over the past several decades, the study of conjunctive queries has occupied a central place in the theory and practice of database systems. In recent years, conjunctive queries have played a prominent role in the design and use of schema mappings for data integration and data exchange tasks. In thi ..."
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Cited by 5 (1 self)
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show that the chase procedure for GLAV mappings (that is, schema mappings specified by sourcetotarget tuplegenerating dependencies) is a local transformation with respect to conjunctivequery equivalence. This means that the chase procedure preserves bounded conjunctivequery equivalence,
Conjunctive queries and mappings with unequalities
"... We study conjunctive queries with unequalities (x ̸ = y) and we identify cases when query containment can still be characterized by the existence of homomorphisms. We also identify a class of GLAVlike database schema mappings with unequalities, for which the chase theorem holds, and thus data excha ..."
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Cited by 2 (1 self)
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We study conjunctive queries with unequalities (x ̸ = y) and we identify cases when query containment can still be characterized by the existence of homomorphisms. We also identify a class of GLAVlike database schema mappings with unequalities, for which the chase theorem holds, and thus data
Homomorphism Preservation on QuasiWide Classes
, 2008
"... A class of structures is said to have the homomorphismpreservation property just in case every firstorder formula that is preserved by homomorphisms on this class is equivalent to an existentialpositive formula. It is known by a result of Rossman that the class of finite structures has this proper ..."
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Cited by 8 (2 self)
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this property and by previous work of Atserias et al. that various of its subclasses do. We extend the latter results by introducing the notion of a quasiwide class and showing that any quasiwide class that is closed under taking substructures and disjoint unions has the homomorphismpreservation property. We
Efficient Approximations of Conjunctive Queries
"... When finding exact answers to a query over a large database is infeasible, it is natural to approximate the query by a more efficient one that comes from a class with good bounds on the complexity of query evaluation. In this paper we study such approximations for conjunctive queries. These queries ..."
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Cited by 5 (3 self)
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When finding exact answers to a query over a large database is infeasible, it is natural to approximate the query by a more efficient one that comes from a class with good bounds on the complexity of query evaluation. In this paper we study such approximations for conjunctive queries. These queries
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