Summary of week four for the course Knowledge Engineering with Semantic Web Technologies 2015 by Harald Sack at OpenHPI.
RDFS Semantics: we need this because there was no formal description of the semantics and then the same querys gave back different results. So you add semantics. Every triple encoded in RDF is a statement and also a RDF-graph is also a statement.
OWL: based on a description logic it consists of classes, properties and individuals (instances of classes)
OWL2 has different flavors with three different dialects (EL, RL, QL), DL (based on description logic and Full (DL is decidable, Full is not). There are different ways to create ontologies. The most important (and shortest) are Manchester Syntax and Turtle.
Classes, properties and individuals in OWL are comparable with the ones in RDFS.
OWL contains NamedIndividuals, which can be introduced directly.
An editor for Ontologies is protege, which can be used as web or desktop application, there are also short courses at their homepage.
Deeper knowledge about OWL in the extra lectures.
Summary of week two for the course Knowledge Engineering with Semantic Web Technologies 2015 by Harald Sack at OpenHPI.
Reification allows you to make reference statements. Therefore a statement also gets an URL. It also allows you to make statements about statements and assumptions about assumptions (e.g. Sherlock Holmes thinks that the gardener murdered the butler).
RDFS or RDF Schema puts this further. It adds more semantic expressivity, you can get more knowledge out of the graph. It is also the simplest of the modelling languages (OWL is another, but will be covered later) and describes vocabularies for RDF. What can we do with it? We can build classes to model structures (Planet is class, satellite is subclass of planet, artificical satellite is subclass…, earth is planet, moon is planet and planet of earth, sputnik is artifical satellite and satellite of earth). From this we can infer some information like: an artificial satellite, is also a satellite. Sputnik is an artificial satellite of earth, so it is also a satellite of Earth!
The rest of the lecture focused on SPARQL. This is the language to query knowledge bases stored in RDF. It is similar to SQL syntactacally, but works somehow different because in RDF we are dealing with graphs. You can use it via an endpoint, which is an RDF database that has a SPARQL protocol layer and gives back HTML. It offers you:
- extraction of data
- construction of new graphs
- update graphs
- logical entailment (inferences)
Results are returned as a triple pattern in turtle + variables. You somehow define a subgraph that the query has to match. Query example at DBPedia. As you can see the syntax is close to SQL, it also offers you filters to reduce the amount of results. If you want to try out SPARQL, OpenHPI recommend the use of Fuseki or Wikidata.