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RDF Dataset

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The RDF data model expresses information as graphs consisting of triples with subject, predicate and object. Many RDF data stores hold multiple RDF graphs and record information about each graph, allowing an application to make queries that involve information from more than one graph.

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A SPARQL query is executed against an RDF Dataset which represents a collection of graphs. An RDF Dataset comprises one graph, the default graph, which does not have a name, and zero or more named graphs, where each named graph is identified by an IRI. A SPARQL query can match different parts of the query pattern against different graphs as described in section 13.3 Querying the Dataset.

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An RDF Dataset may contain zero named graphs; an RDF Dataset always contains one default graph. A query does not need to involve matching the default graph; the query can just involve matching named graphs.

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The graph that is used for matching a basic graph pattern is the active graph. In the previous sections, all queries have been shown executed against a single graph, the default graph of an RDF dataset as the active graph. The GRAPH keyword is used to make the active graph one of all of the named graphs in the dataset for part of the query.

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Examples of RDF Datasets

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The definition of RDF Dataset does not restrict the relationships of named and default graphs. Information can be repeated in different graphs; relationships between graphs can be exposed. Two useful arrangements are:

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  • to have information in the default graph that includes provenance information about the named graphs
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  • to include the information in the named graphs in the default graph as well.
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Example 1:```turtle

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Default graph

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@prefix dc: http://purl.org/dc/elements/1.1/ .

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http://example.org/bob dc:publisher \"Bob\" .\nhttp://example.org/alice dc:publisher \"Alice\" .

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\n```turtle\n# Named graph: http://example.org/bob\n@prefix foaf: <http://xmlns.com/foaf/0.1/> .\n\n_:a foaf:name \"Bob\" .\n_:a foaf:mbox <mailto:bob@oldcorp.example.org> .\n
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# Named graph: http://example.org/alice\n@prefix foaf: <http://xmlns.com/foaf/0.1/> .\n\n_:a foaf:name \"Alice\" .\n_:a foaf:mbox <mailto:alice@work.example.org> .\n
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In this example, the default graph contains the names of the publishers of two named graphs. The triples in the named graphs are not visible in the default graph in this example.

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Example 2:

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RDF data can be combined by the RDF merge [RDF Semantics] of graphs. One possible arrangement of graphs in an RDF Dataset is to have the default graph be the RDF merge of some or all of the information in the named graphs.

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In this next example, the named graphs contain the same triples as before. The RDF dataset includes an RDF merge of the named graphs in the default graph, re-labeling blank nodes to keep them distinct.

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# Default graph\n@prefix foaf: <http://xmlns.com/foaf/0.1/> .\n\n_:x foaf:name \"Bob\" .\n_:x foaf:mbox <mailto:bob@oldcorp.example.org> .\n\n_:y foaf:name \"Alice\" .\n_:y foaf:mbox <mailto:alice@work.example.org> .\n
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# Named graph: http://example.org/bob\n@prefix foaf: <http://xmlns.com/foaf/0.1/> .\n\n_:a foaf:name \"Bob\" .\n_:a foaf:mbox <mailto:bob@oldcorp.example.org> .\n
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# Named graph: http://example.org/alice\n@prefix foaf: <http://xmlns.com/foaf/0.1/> .\n\n_:a foaf:name \"Alice\" .\n_:a foaf:mbox <[mailto:alice@work.example](mailto:alice@work.example)> .\n
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In an RDF merge, blank nodes in the merged graph are not shared with blank nodes from the graphs being merged.

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