e., a string,
an integer, but also a text, an image, a sound) and do not exist independently of their
parent complex node. This model represents the valid time dimension.
In Amagasa et al. (2001), the authors propose a logical data model for representing
histories of XML documents. The proposed model is an extension of the XPath data
model (World Wide Web Consortium, 2005a), with a label on edges expressing valid
time. The result of a general XPath expression may be a selection of nodes from the
input documents, or an atomic value, or more generally, any sequence allowed by
282 Combi & Oliboni
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the data model. In the proposed extension, nodes can be selected also with respect
to their valid time.
In Dyreson et al. (1999), the authors propose a graph-based model which uses
labeled graphs to represent semistructured databases. In the defined graphs, each
edge label is composed by a set of descriptive properties (e.g., name, transaction
time, valid time, security properties of relationships): a property can be present in
an edge and missing in another one. This proposal is very general and extensible:
any property may be used and added to adapt the model to a specific context. In
particular, the model allows one to represent temporal aspects and to consider
only a temporal dimension or multiple temporal dimensions: to this regard, some
examples of constraints which need to be suitably managed to correctly support
semantics of the time-related properties are provided, both for querying and for
manipulating graphs.
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