2 8 Combi & Oliboni
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Introduction
In recent years the database community has proposed flexible data models to represent
semistructured information. Semistructured data have no absolute schema fixed
in advance. The structure may be irregular or incomplete (Abiteboul, 1997).
In the literature there are a number of approaches in which labeled graphs are
used to represent semistructured data (Comai, Damiani, Posenato, & Tanca, 1998;
Damiani, Oliboni, Tanca, & Veronese, 1999; Papakonstantinou, Garcia-Molina, &
Widom, 1995). These models organize data in graphs where nodes denote objects
or values, and edges represent relationships between them.
In the semistructured data context, the eXtensible Markup Language (XML) (World
Wide Web Consortium, 1998) is spreading out as a standard for representing, exchanging,
and publishing semistructured information (Abiteboul, Buneman, &
Suciu, 2000), making information ???self-describing,??? that is it is possible there is
no separate description of the type or structure of data.
A data warehouse is a repository of data coming from different and heterogeneous
data sources. This means that data stored in a data warehouse are semistructured
in nature, because in different documents the same information can be represented
in different ways, and moreover, the document schemata can be available or not.
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