Archive for April, 2009

Webdam at EDBT Summer School 2009

April 30th, 2009
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The first results of Webdam will be presented at the 9th EDBT Summer School (August 31th- September 4th) during the first slot of the Programme dedicated to “First results from the ERC-funded projects” .

In case Serge is not available to go there for presenting Webdam (though he is announced as a speaker :-)), Marie-Christine Rousset will make the presentation. As a matter of facts, she will also give a tutorial on “semantic oriented data spaces”.

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Werner Nutt visiting Webdam

April 16th, 2009
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Werner Nutt is visiting Webdam from Monday 20 April to Friday 24 April 2009. He is a professor at the Faculty of Computer Science at the Free University of Bozen-Bolzano.
He will present his work on containment of conjunctive queries over databases with null values Friday at 2:00pm in the meeting room G008.

Title : Containment of Conjunctive Queries over Databases with Null Values (Joint work with Carles Farre, Ernest Teniente, and Toni Urpi, UPC Barcelona)

Summary : Intuitively, one query “contains” another query if it is more general than the other query. Query containment is a key topic in database theory, which was originally motivated as a foundation of query optimisation, but has also other applications, such as integrity checking and information integration.
The containment problem has been studied for many different types of queries. The work so far, however, has never considered the effect of SQL style null values, although they are ubiquitous in real world data and queries have to process them.
In this talk we discuss in which ways null-containment, that is, containment in the presence of null values, differs from the non-null case. We consider conjunctive queries, which are essentially single block SQL queries. We show that null-containment, as in the non-null case, is NP-complete for boolean queries, which return only yes or no, while the situation becomes different from the classical case for queries that output data. We also discuss sufficient, but non-necessary criteria for null-containment, special cases with polynomial complexity, the effect of “is null” tests, as well as null-containment for richer classes of queries where comparisons or unions are allowed.

Short Bio : Werner Nutt is a professor at the Faculty of Computer Science at the Free University of Bozen-Bolzano in the Italian Alps (since 2005). He obtained a PhD and a habilitation from the university of Saarbruecken, was a visiting associate professor in Jerusalem (1997-2000) and a reader at Heriot-Watt University in Edinburgh (2000-2005). His research interests are in Artificial Intelligence and Databases, with a focus on Description Logics, Information Integration and Incomplete Information.

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Bruno Marnette visiting Webdam

April 16th, 2009
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Bruno Marnette is visiting Webdam Thursday 16 April 2009. He is a phD student at the computing laboratory of the Oxford University.
He will present his work on schema mappings Thursday at 10:30am at ENS Cachan in LSV-library.

Title : Schema-Mappings: From Termination To Tractability

Summary : Data-Exchange is the problem of creating automatically a new database while integrating the information encoded in (1) a given source database and (2) a high-level specification called schema-mapping. After investing the general properties of (generalised) schema-mappings and their semantics, I will introduce in this talk a new sufficient condition TOC for polynomial data-complexity. This criteria TOC generalizes strictly and substantially the best previously-known criteria called Weak-Acyclicity and relies on the termination of (a refinement of) a known procedure called Oblivious Chase. While TOC is RE-complete, I will finally present a more restrictive criteria SwA (for Super-weak Acylicity) that can be decided in polynomial time while generalizing already substantially the notion of Weak Acyclicity.

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