Introduction to RDF and Turtle Syntax

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Questions and Answers

What is the primary role of blank nodes in RDF?

They represent resources without a URI. (correct)

They serve as unique identifiers for every resource.

They symbolize relationships between literals.

They are used exclusively for numeric literals.

In which position are literals not allowed in an RDF triple?

Subject position

Predicate position (correct)

Object position

None of the above

Which of the following is NOT a type of serialization in RDF?

N-Triples

XML-Document (correct)

Turtle

JSON-LD

How do you denote a namespace prefix in Turtle syntax?

By using the '@prefix' declaration Signup and view all the answers

What is true about the predicate properties in RDF?

They are useful for modeling and create a hypergraph structure. Signup and view all the answers

In Turtle syntax, how are comments made?

With the '#' character Signup and view all the answers

What does the shorthand in Turtle allow when declaring statements?

Subjects and predicates can be combined into single declarations. Signup and view all the answers

How are literals handled in Turtle syntax?

They can be expressed without quotes for numbers and Booleans. Signup and view all the answers

What is the primary role of properties in RDF Schema?

They are treated as first-class citizens. Signup and view all the answers

What is one of the key distinctions made in RDF Schema regarding data?

The separation of schema and actual data. Signup and view all the answers

Which statement is true regarding the expressiveness of RDFS?

RDFS is not very expressive. Signup and view all the answers

What do entailment rules in RDFS accomplish?

They express operations to produce new facts from formulas. Signup and view all the answers

Which vocabulary is specifically designed to facilitate search engine indexing?

Schema.org Signup and view all the answers

What is a core advantage of using HTTP URIs in RDF graphs?

They are unique throughout the web, which helps avoid name clashes. Signup and view all the answers

Which statement accurately describes the nature of triples in RDF?

They make relationships explicit and are elements in their own right. Signup and view all the answers

What operation is used to merge two RDF graphs?

Taking their union. Signup and view all the answers

Why are RDF graphs described as versatile?

They can utilize a single format for various data relationships. Signup and view all the answers

What is a defining characteristic of RDF graphs compared to other data formats?

They consist of a set of triples, which allows for specific operations. Signup and view all the answers

What is a model of a knowledge base?

An interpretation that is a model of all its triples Signup and view all the answers

Which element is NOT part of a triple in the RDF?

Property Signup and view all the answers

What does RDF primarily facilitate?

Data merging across differing schemas Signup and view all the answers

What is the role of URIs in RDF?

Identify resources but are not the resources themselves Signup and view all the answers

Which of the following statements about URIs is incorrect?

URIs and URLs are the same. Signup and view all the answers

What does a literal in RDF represent?

A data value such as a string or integer Signup and view all the answers

Which principle is NOT part of the Linked Data principles?

Data should be accessible only through proprietary software. Signup and view all the answers

How does RDF extend the linking structure of the web?

Through the application of URIs for naming relationships Signup and view all the answers

What does 'Principal 4' emphasize regarding data and models?

A shared model and defined formal semantics are required. Signup and view all the answers

What is the default data type assumed if none is specified for a literal in RDF?

xsd:string Signup and view all the answers

What does OWL primarily build upon?

Description Logics Signup and view all the answers

What is a necessary condition in OWL?

It allows for subclass relations to be inferred. Signup and view all the answers

In RDF, what is a triple?

A relationship among three entities. Signup and view all the answers

What is the purpose of the owl:equivalentClass property?

To infer class equivalence. Signup and view all the answers

What does the rdfs:domain property specify?

The class of instances that can use that property. Signup and view all the answers

What does the owl:hasSelf property indicate?

Members of a class have self-referencing relations. Signup and view all the answers

During the process of data integration, what is the first step?

Build an ontology in Protégé. Signup and view all the answers

What kind of queries can be constructed using SPARQL?

Both select and insert queries. Signup and view all the answers

Which of the following is a characteristic of OWL classes?

Classes can be defined by describing conditions on their members. Signup and view all the answers

What is the role of owl:hasValue?

To set specific values for class instances. Signup and view all the answers

Which description best fits the concept of punning in OWL?

Using classes as instances. Signup and view all the answers

What is the significance of using owl:maxQualifiedCardinality?

It limits the maximum number of relationships to specific instances. Signup and view all the answers

What criterion must be met for a set of triples to be derived from a knowledge graph?

It must be a subgraph of the knowledge graph. Signup and view all the answers

Which technology is essential for ensuring data is interconnected and accessible on the web?

Uniform Resource Identifier (URI) Signup and view all the answers

Which of the following aspects does Principal 4 of Linked Data emphasize?

The necessity for a shared model and defined formal semantics. Signup and view all the answers

What indicates that an interpretation functions as a model of a knowledge base?

It must model all triples within the knowledge base. Signup and view all the answers

For interoperability in linked data, which of the following is necessary regarding shared formats?

Data should be represented in known data formats for querying. Signup and view all the answers

In the context of propositional logic, what is the role of semantic entailment?

To determine if left true values guarantee the truth of another formula Signup and view all the answers

Which option correctly describes the nature of a declarative sentence in propositional logic?

It must be either true or false, not both. Signup and view all the answers

What do truth value semantics express in the context of propositional logic?

They formalize the relationship between syntax and truth conditions. Signup and view all the answers

Which characteristic is essential for an effective knowledge graph logic?

Unambiguity in the language used Signup and view all the answers

What is the significance of using prefix notation in propositional logic?

It simplifies the evaluation of logical expressions by placing operators before their operands. Signup and view all the answers

What are entailment rules used for in RDF Schema?

To produce new facts from existing formulas Signup and view all the answers

Which statement accurately describes a characteristic of RDFS?

It utilizes reserved symbols to define schema and properties Signup and view all the answers

Which vocabulary is known for facilitating the linking of data about documents?

Dublin Core Signup and view all the answers

How does RDFS’s lack of formal semantics impact data interpretation?

It renders the meaning of the web of data ambiguous. Signup and view all the answers

What does RDFS's rdfs:subPropertyOf indicate?

It establishes a hierarchy of properties. Signup and view all the answers

What is a critical aspect that must align with the intended visualization in ontology engineering?

Describing the domain and scope Signup and view all the answers

Which option best represents the requirements for using external data sources in creating the website's ontology?

At least two sources, convertible to RDF formats like CSV or XML Signup and view all the answers

Which statement most accurately describes the ontologies selected for the website's design?

The choice should reflect a balance between simplicity and complexity Signup and view all the answers

What is a key step in presenting the idea of data visualization during the milestone pitch?

Detail the conceptualization of the domain and its visual aspects Signup and view all the answers

In ontology engineering, why is it important to define the methodology used to construct the ontology?

It helps in understanding the implementation and interrelations Signup and view all the answers

What key feature allows RDF to handle structured and semi-structured data from different applications?

It merges data even when underlying models differ. Signup and view all the answers

What is the role of literals in RDF triples?

They represent data values and must have a data type. Signup and view all the answers

Why is it important to distinguish between URIs and URLs in RDF?

A URI can refer to a resource, while a URL is specifically for web locations. Signup and view all the answers

How is the data type assigned to literals in RDF by default?

It defaults to xsd:string if no type is provided. Signup and view all the answers

What representation is used to express all information in RDF?

Triples comprising a subject, predicate, and object. Signup and view all the answers

What aspect do Internationalized Resource Identifiers (IRIs) enhance over traditional URIs?

IRIs allow the use of Unicode characters. Signup and view all the answers

Which of the following best describes the graph view of RDF?

It forms a directed and labeled graph representing relationships. Signup and view all the answers

What defines a URI's relationship to the resource it identifies?

A URI refers to a resource, but is not the resource. Signup and view all the answers

Study Notes

Blank nodes

A resource without a URI (Uniform Resource Identifier)
Used when a resource is unknown or has no natural identifier.
In logical terms, they're like existential quantifiers.
Need to extend the syntax of formal semantics to include them.

Triple Grammar

Literals and Blank Nodes may not appear in every position of a triple.
Literals are just values (no relationships allowed from literals).
Blank nodes in the predicate position are confusing because they lack meaning.

Properties of Predicates

Useful in modelling.
They transform a graph into a hypergraph.
Information focuses on the general predicate, not its specific use in a triple.

Serializations

RDF/XML: Historical and outdated.
Turtle: A convenient and human-readable/writable syntax.
N-Triples: One unabbreviated triple per line for easy parsing.
Graph-aware formats: Trix, TriG, JSON-LD, and N-Quads.
RDFa: Annotations on HTML elements.

Turtle Syntax

Comments start with '#' character.
Full URIs are surrounded by '<' and '>'.
Statements are triples, terminated by a period.
Use 'a' to abbreviate rdf:type.
Namespace prefixes are declared with '@prefix'.
A default namespace can be declared with '@prefix :'.
Literal values are enclosed in double quotes.
Language or type information can be included.
Numbers and Booleans can be written without quotes.
Shorthand options:
- Instead of repeating the subjects, the statements share a subject with ';'.
- Instead of repeating the subjects and predicates, the statements share the subject and predicate with ','.
- Two ways to write Blank Nodes: - Written with underscores: _... - Written with brackets: [...]

Semantics

An interpretation is a model of a knowledge base if it's a model of all its triples.
A triple is entailed by a knowledge base if it's true in all models of the knowledge base.
A model is an example that's true with reference to the interpretations.

Linked Open Data (LOD)

Also known as LOP.
Open data: about licenses to allow reuse.
Linked data: about technology for interoperability.

Principles of Linked Data

Principles 1-3: The data needs to be connected and accessible on the web.
Principle 4: We need a shared model and defined formal semantics for predictable inferencing.

Technology for Linked Data

Uniform Resource Identifier (URI)
The HyperText Transport Protocol (HTTP)
Namespaces
Resource Description Framework (RDF)
SPARQL
OWL
Ontology Editors

RDF (Resource Description Framework)

A query language to describe things.
Extends the linking structure of the web, using URIs to name the relationship between things, as well as the two ends of the link (a triple).
Allows structured and semi-structured data to be mixed, exposed, and shared across different applications, acting as a data model for data interchange on the web.
Facilitates data merging even if the underlying schemas or models differ.
The links form a directed and labeled graph.
Graph view is often used in visual explanations.
Components:
- All information is expressed as a triple: two-placed predicates.
- Triples consist of a subject, a predicate, and an object (s, p, o). - Subject: the first part of the triple. - Predicate: the middle part of the triple. - Object: the last part of the triple. - Together, the three words form a fact.
- Triple = statement or fact or axiom.
- Elements of a triple are either URI references, blank nodes, or literals.
Resources are identified by URIs.

URIs

They only refer to a resource, they are not the resource.
Multiple URIs can denote the same resource (one resource can have multiple identifiers).
Internationalized Resource Identifiers (IRIs) are URIs that allow Unicode characters.
RDF-URI != URL.
Use '@' to define URLs/URIs in the code for brevity.

Literals

Represent "literal" data values.
Have a data type (string or integer).
Data types are also resources, referenced via URIs.
Default: xsd:string if no data type is specified.
Language of a string can be specified using a language tag.
Always go at the end of a triple.

Graphs

A set of triples.
A graph that contains two triples.
Many RDF graphs have URIs, making them hypergraphs rather than traditional graphs.
Often represented as a directed labelled graph.

Why Use HTTP URIs?

They have a global scope and are grounded in society.
They are unique throughout the web, helping to avoid name clashes.
They are also addresses, exploiting well-functioning web browsing machinery.
They track data by following the resource identifiers found in triples.

Why Triples?

Any information format can be transformed into triples (simple).
Relationships are made explicit and are elements in their own right, unlike database columns and binary predicates.
The predicate is an element in the triple and can be described in RDF.
"Self-documenting".

Why Graphs?

A single, highly versatile format.
Basic set operations are well-defined since RDF Graphs are just a set of triples.
Merging two RDF Graphs: Take their union.
Extending an RDF Graph: Written with brackets [...].

RDF Schema (RDFS)

Observations:
- Properties are first-class citizens.
- No strict distinction between … (…).
Additional Stuff:
- Specify human-readable labels for a resource.
- Comment on a resource.
- Refer to another resource.
Key Relationships:
- rdf:type rdfs:domain --> rdfs:Resource
- rdf:type rdfs:range --> rdfs:Class
- rdfs:subPropertyOf rdfs:range --> rdf:Property
- rdfs:label rdfs:range --> rdfs:Literal
Summary:
- Without formal semantics, the web of data is meaningless.
- Distinction between classes, properties, and instances (schema vs. data).
- RDFS reserved symbols: rdfs:class, rdfs:subClassOf, rdfs:domain, rdfs:range, …
- Entailment rules are expressed using reserved symbols.
- Inferencing is the application of entailment rules to formulas to produce new facts.
- RDFS is not very expressive.

RDFS and Other Vocabularies

RDF Vocabulary:
- The RDF vocabulary and RDFS reserved terms needed for the data model.
- Explained previously in the notes.
Friend of a Friend (FOAF)
- Not a formal language/semantic.
Dublin Core (DC and DCTerms)
- About documents.
Schema.org
- Used by search engine companies to index their datasets.
Dbpedia Ontology
- Originally from Wikipedia.
WordNet
- Describes words.
Thesauri
- Standard terminology in a particular domain.
MeSH
- Medical Subject Headings.
SKOS
- Vocabulary for Thesaurus modeling.

RDF, RDFS, and OWL have formally-defined semantics.
- With reference to graphs (RDF).
- With reference to sets (RDFS/OWL).
FOAF and others have informal semantics.
- Defined in textual descriptions.
- Defined in their usage online.

Example: Domain (Book)

Instances:
- To Kill A Mockingbird (Harper Lee)
- 1984 (George Orwell)
- The Handmaid's Tale (Margaret Atwood)
Classes:
- Author
- Genre
- Book
- Novel
Properties:
- hasAuthor
- hasGenre
- hasEbook
Triples:
- TKAMB hasAuthor Harper Lee
- Harper Lee a (rdf:type) Author
- THMT a (rdf:type) Novel
- hasAuthor rdfs:domain Book
- THMT a (rdf:type) Book
- 1984 hasGenre sci-fi

OWL (Web Ontology Language)

Provides features for defining classes and properties.
Built on description logics (DL).
Extension of RDFS semantics and syntax.
Restriction of RDFS semantics and syntax.
Strict separation of instances, classes, and properties.
However, there is punning.

OWL Class Restrictions

A class is a set of individuals.
Examples:
- owl:equivalentClass
- owl:complementaryClass
- owl:unionOf
- ...
owl:Restriction:
- An owl:Class defined by describing conditions on the individuals it contains.
- It describes the class of things that … (the condition).
Two types of restrictions:
- Necessary conditions:
  - Members of the class must have this property, object, or condition.
  - Uses rdfs:subClassOf to infer subclass relations and property values of known class members.
- Necessary and sufficient conditions:
  - Anything that is a property of this condition is automatically a member of this class.
  - Uses owl:equivalentClass to infer class equivalence and class membership of individuals.
Properties to describe the conditions:
- Existential -> owl:someValuesFrom:
  - All members of a class have at least some value from the specified class.
- Universal -> owl:allValuesFrom:
  - All members of a class have only values from the specified class (all).
- Specific value -> owl:hasValue:
  - All members of a class have this (specific) instance as value.
- Cardinality -> owl:minQualifiedCardinality, owl:maxQualifiedCardinality, owl:qualifiedCardinality:
  - All members of a class have exactly/min/max n values (from the specified class).
- Is reflexive -> owl:hasSelf (Boolean):
  - All members of a class have a relation with themselves (local reflexivity).
Be careful of naming the range of property restrictions:
- The range should not be the same for other properties, unless it should be.
Assertions:
- Identity and negation: State that two individuals are different or the same. You can say that a property does not hold between two individuals.

Using Protégé for class axioms with OWL

Make Entities: Drag the subclasses.
Start Reasoner to get implied triples: Install the needed packages (pellet?).
Save as a ttl file in case it crashes.
At least x: min x.
Do the conditions in equivalent to.

Punning

This implies a combination of different classes or properties, creating a multifaceted meaning.

Data Integration

Steps:
- Build the ontology in protégé.
- Import the turtle file into GraphDB.
- Get some interesting data in CSV.
- http://prefix.cc/: Look for prefix URLs.
- SPARQL:
  - Construct: Get triples.
  - Insert: Insert the triples into the knowledge graph.
  - For debugging the query or finding the problem: Go to www.dbpedia.org/sparql.

Federated Queries

Surface keywords: Parts of the SPARQL query that have a "service" and "filter".
Add multiple services and filters to the query.

Help

https://hub.compute.vu.nl/lander
Dbpedia and Wikidata: Practice writing queries for the final project (numeric values).
- http://query.wikidata.org/sparql
Use Wikidata numeric identifiers in the SPARQL query.

Final Project

Deadlines:
Milestone 1: Oct 13.
- Can be reused in the final report.
- Do not have to be complete, but provide good feedback for more complete data.
Milestone Pitch: Oct 17.
- Everything so far is graded (for the final report).
- More like a pass/fail grade with feedback.
Final report: Oct 27.
- 8-12 pages.
- Can go below/over the limit if it's complete.
- The visuals must be in the appendix.
Notes:
- Goals:
  - Design and implement a data analysis pipeline with KAD technology (OWL, RDF).
  - Document the process and present the results.
- Deadline 1: Milestone:
  - What kind of question do you plan to answer?

Concepts & Assignments

Concepts are abstractions or generalizations from experience, or transformations of existing ideas.
An assignment is a model of a knowledge base if it models all its axioms.

Propositional Logic

Declarative sentences or propositions are statements that are either true or false.
Propositional logic has syntax, semantics, and a calculus.
Connectives include: and, or, either or, not, if then, if and only if.
Constructs include: for all, there exists.
Infix notation: q -> p.
Prefix notation: -> , [q], [p].
Formulas of propositional logic express declarative statements.

Semantic Entailment

A formula is semantically entailed if every valuation that makes all formulas true also makes the second formula true.
The core of logical reasoning.
A counterexample is a false example.
If there exists a counterexample, then there is no entailment.

Simple Knowledge Graph Logic

We need an unambiguous language.
Triples transform data to formal syntaxes in a web of data.
A set of triples is entailed by a knowledge graph if it's a subgraph of the knowledge graph.

Knowledge Graphic Logic & KGs on The Web

An interpretation is a model of a knowledge base if it's a model of all its triples.
A triple is entailed by a knowledge base if it's true in all models of the knowledge base.
A model is an example or a triple that is true with reference to the interpretations.

Linking Data: Linked (Open) Data

Linked (Open) Data, AKA LOP.
Open data: about licenses to allow reuse.
Linked data: about technology for interoperability.
Principals 1-3: Data needs to be connected and accessible on the web.
Principal 4: Shared models and defined formal semantics are needed for predictable inferencing.

Technology: Standards, Standards, Standards (Query Languages)

Uniform resource identifiers (URI).
The hypertext transport protocol (HTTP).
Namespaces.
Resource description framework (RDF).
SPARQL (sparkle).
OWL.
Ontology editors

RDF

Resource description framework.
A query language to describe things.
Extends the linking structure of the web to use URIs to name the relationship between things.
Allows structured and semi-structured data to be mixed, exposed, and shared across applications.
Facilitates data merging even if underlying schemas/models differ.
The links form a directed and labeled graph.
Components:
- All information is expressed as triple: two-placed predicates.
- Triples consist of a subject, a predicate, and an object (s, p, o).
  - The first one = subject.
  - The middle one = predicate.
  - The last one = object.
  - The three words = fact.
- Triple = statement or fact or axiom.
- The elements of an RDF triple are either URI references, blank nodes (variables), or literals (string parts or values).
Resources are identified by URIs.