[SOLVED] How do linked open data datasets linked together?

How do linked open data datasets linked together?

I'm learning about RDF databases and the concept of "Linked Open Data (LOD)". According to what Wiki saids about LOD, many open-accessible datasets are published online and some of the resources in these datasets are interlinked with each other under a certain protocol. But it seems the protocol is not clearly defined. I'm wondering how are resources from different datasets linked with each other? Are there some standardized vocabularies and semantics for this purpose, or the linking semantics is defined within a dataset itself?

Solution

The semantics of the links get defined by a vocabulary (or ontology). This could be a vocabulary defined by the linking dataset, by the linked dataset, or by a third-party.

In that context, it’s of no relevance whether it’s an "external" link (linking to a different dataset, possibly under a different namespace/domain) or an "internal" link (linking within the same dataset), nor whether the linked resource even provides RDF. It’s just a reference to some URI either way.

Two well-known properties for linking instances are owl:sameAs ("determines that two given individuals are equal") and rdfs:seeAlso ("to indicate a resource that might provide additional information about the subject resource"). And there are of course countless domain-specific properties, e.g. foaf:knows ("A person known by this person") and schema:author ("The author of this content").

Example

(omitted the prefix declarations for brevity)

The DBpedia entry which represents the musician Moby says that https://dbpedia.org/resource/Moby and http://musicbrainz.org/artist/8970d868-0723-483b-a75b-51088913d3d4 represent the same individual:

dbr:Moby 
  owl:sameAs <http://musicbrainz.org/artist/8970d868-0723-483b-a75b-51088913d3d4> .

In DBTune, it says about Moby:

db:artist/8970d868-0723-483b-a75b-51088913d3d4 
  owl:sameAs <http://www.bbc.co.uk/music/artists/8970d868-0723-483b-a75b-51088913d3d4#artist> ;
  owl:sameAs dbr:Moby .

So four datasets are linked (DBpedia, MusicBrainz, DBTune, BBC Music), in this case by linking four URIs which all represent the musician Moby.