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Semantics enriches data with meaning. This meaning is stored in semantic graph databases. Graph databases open up new possibilities and perspectives for developers in many ways, and not just when it comes to modeling, classes, and their properties. In contrast to the well-known SQL and NoSQL databases with their tables and columns, listings and fields, graph databases extend the familiar view of schemas, objects, and inheritance with new features such as metadata, inference, and linking multiple graph databases to extensive knowledge graphs. With the appropriate tools, data can be linked to a semantic network. This is not new. Tim Berners-Lee coined the term "semantic web" as early as 2001. The W3C extended it in 2004 with standards such as OWL (Web Ontology Language, extended in 2012 with OWL2) and made it practical.

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A collection of triples is also referred to as a triple store. At the lowest level, a semantic graph database is such a triple store. How the triples are managed internally as efficiently as possible is up to the database vendor. For import into and export from files, the RDF triples are represented in various formats. These can be, for example, Turtle, Trig, N-Triples, JSON-LD, or even RDF/XML, with their respective advantages and disadvantages. While Turtle (.ttl) is popular because of its compactness and easy readability, Trig (.trig) is more suitable for backup and restore of knowledge graphs since the information about subgraphs is also serialized and deserialized. As expected, JSON-LD is increasingly found in JavaScript environments and RDF/XML more in the Java world. However, in the spirit of interoperability, semantic graph databases such as GraphDB from Ontotext have import and export functions for all important formats.

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RDFS

The abbreviation RDFS stands for Resource Description Framework Schema [5], a semantic extension of the RDF vocabulary specifically for data modeling. It contains mechanisms especially for grouping resources and their relationships to each other. It is comparable to the class system known from object-oriented programming (OOP), but with an essential and important extension: While in OOP it is defined which properties a class has and may have, an RDF schema can specify for properties to which class an individual is automatically assigned if it contains or references the property in question. This includes support for subClassOf and subPropertyOf to organize classes and properties hierarchically. RDFS plus, finally, is an extended version of RDFS that supports symmetric, inverse, and transitive properties. These new concepts, in contrast to OOP, will be explained in more detail below. Many of the RDFS and RDFS plus components are also part of the even more expressive Web Ontology Language (OWL).

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