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Dec 5, 2020

RDF* and the onset of Linked Data* and the Semantic Web*

The evolution of RDF and the related technologies fuelled by the need to make statements about statements.

RDF* ("RDF star") is an extension of the RDF standard that adds support for embedded (or nested) statements. RDF* statements that include nested statements, or statements about statements, are extensions of basic RDF triples and can be thought of as meta-statements or metadata in relation to the embedded statements.

In RDF*, the role of meta-statements is similar to that of edge properties in property graphs: statements about statements add additional information about existing edges in the graph and therefore act as statement-level annotations. This provides a more convenient way of modelling and storing complex relationships and is useful for expressing various kinds of metadata such as information about data provenance.

The limitations of conventional RDF

In traditional RDF, statements can only connect two entities (nodes) or one node and one literal value at a time. Because the subject of an RDF triple is always a node, the standard does not allow "chaining" of statements or adding a statement that "branches out" from another statement. This can be seen as a limitation of the traditional RDF data model, as it does not provide the means to simply refer to an existing statement within another statement. For example, while the fact that Alice is friends with Bob can be easily modelled using a single RDF triple:

:Alice :friendsWith :Bob .

additional information about their friendship, e.g. its start date, can only be "attached" when the friendship "exists" as a node:

:friendship0 :person :Alice ;
             :person :Bob .
:friendship0 :startDate "2020" .

Meet RDF*

RDF* natively supports meta-statements, or statements about statements, and can be used to store the start date of Alice and Bob's friendship more succinctly compared to RDF:

<<:Alice :friendsWith :Bob>> :startDate "2020" .

There are many kinds of metadata that, thanks to RDF*, can be conveniently captured at the level of individual statements. These include various score values such as edge weights as well as statement-level provenance information:

:Alice :friendsWith :Bob .
<<:Alice :friendsWith :Bob>> :relevance "10" .
<<:Alice :friendsWith :Bob>> :dateCreated "2005" .
<<:Alice :friendsWith :Bob>> :source <> .

Linked Data* and the Semantic Web*

Just as RDF* extends RDF to include support for embedded statements, Turtle* and SPARQL* represent the evolution of the corresponding data serialisation and querying technologies used with RDF/RDF*.

For example, in Turtle*, embedded statements are written inside a pair of << >>, as in the following example:

<<:Alice :friendsWith :Bob>> :startDate "2020" .
:Alice :knows <<:Bob :friendsWith :Carol>> .

Interestingly, since RDF* supports multiple levels of nesting, a statement about a statement about a statement (statement meta-metadata) can be written in Turtle* as follows:

<<<<:Alice :friendsWith :Bob>> :startDate "2020">> :source <> .

See also
The building blocks of OWL
What makes up OWL ontologies and how do they support logical inference?
Why federation is a game-changing feature of SPARQL
SPARQL federation is an incredibly useful feature for querying distributed RDF graphs.
Let's explore the Nobel Prize dataset
An overview of the official Nobel Prize Linked Data dataset with some example SPARQL queries.
Towards more linked lexicographical data: Lexemes on Wikidata
A glimpse into the meaning and other properties of words described with structured and linked data.
Data exploration on linked COVID-19 datasets
An overview of the available RDF datasets and discovery tools for COVID-19.
Linked data for the enterprise: Focus on Bayer's corporate asset register
An overview of COLID, the data asset management platform built using semantic technologies.
What does a knowledge engineer do?
An overview of knowledge engineering and the core competencies and responsibilities of a knowledge engineer.
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