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DBPL '01 Revised Papers from the 8th International Workshop on Database Programming Languages
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VLDB '05 Proceedings of the 31st international conference on Very large data bases
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VLDB '03 Proceedings of the 29th international conference on Very large data bases - Volume 29
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ADC '08 Proceedings of the nineteenth conference on Australasian database - Volume 75
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The VLDB Journal — The International Journal on Very Large Data Bases
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ESWC 2009 Heraklion Proceedings of the 6th European Semantic Web Conference on The Semantic Web: Research and Applications
Information Systems
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APWeb/WAIM'07 Proceedings of the joint 9th Asia-Pacific web and 8th international conference on web-age information management conference on Advances in data and web management
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DASFAA'07 Proceedings of the 12th international conference on Database systems for advanced applications
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ISWC'10 Proceedings of the 9th international semantic web conference on The semantic web - Volume Part I
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WISE'10 Proceedings of the 11th international conference on Web information systems engineering
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ACM SIGMOD Record
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Expressive languages for selecting groups from graph-structured data
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Flexible queries facilitate, in a novel way, easy and concise querying of databases that have varying structures. Two different semantics, flexible and semiflexible, are introduced and investigated. The complexity of evaluating queries under the two semantics is analyzed. Query evaluation is polynomial in the size of the query, the database and the result in the following two cases. First, a semiflexible DAG query and a tree database. Second, a flexible tree query and a database that is any graph. Query containment and equivalence are also investigated. For the flexible semantics, query equivalence is always polynomial. For the semiflexible semantics, query equivalence is polynomial for DAG queries and exponential when the queries have cycles. Under the semiflexible and flexible semantics, two databases could be equivalent even when they are not isomorphic. Database equivalence is formally defined and characterized. The complexity of deciding equivalences among databases is analyzed. The implications of database equivalence on query evaluation are explained.