1. +
RDFS
A little semantics goes a
long way
Mariano Rodriguez-Muro,
Free University of Bozen-Bolzano

2. +
Disclaimer

License


A few examples from these slides has been taken from


This work is licensed under a
Creative Commons Attribution-Share Alike 3.0 License
(http://creativecommons.org/licenses/by-sa/3.0/)
Semantic Web for the working Ontologist. Chapter 6.
Some of the slides on the use of taxonomies are based on:

http://info.earley.com/webinar-replay-business-value-taxonomy-aug2012

3. +
Reading material

Semantic Web for the working Ontologist. Chapter 6
http://proquest.safaribooksonline.com/book/-/9780123859655

4. +
Motivation

Ontologies

RDFS ontologies

Overview

Annotations

5. +
Motivation

6. +
Motivation

Motivations for semantic
technology

Making the web machine
understandable

Expressing knowledge


However, points 2 and 3 are not
possible with the technologies
seen so far

RDF doesn’t define
vocabularies, and

Different datasets may use
different URI’s to represent the
same kind of data
Reasoning with knowledge

7. +
Motivation

Agreement in RDF graphs concerns only the data model and the use
of URI as identifiers

No semantics!

8. +
Ontologies and Ontology
Languages

9. +
What is an ontology language?

Specification of valid “axioms”

Specifications of vocabularies with “predefined” meaning in axioms

Informal: Topic Maps, UML diagrams

Formal examples: Predicate Logic, First Order Logic

Semantic Web examples:



RDFS
SWRL,
OWL

Different languages have different expressive power

Axioms allow to produce “inferences”

The more expressive power, more complex and costly the inferences

10. +
What is an ontology?

Collections of “axioms”

Describe the meaning of the vocabulary of a domain (e.g., an
area of expertise)

Expressed in an Ontology Language

Valuable on their own as knowledge repositories

In combination with data valuable to implement complex
behavior with little or no coding

11. +
Example: Schema.org

Schema.org IS a simple ontology

Organizes terms in hierarchies with
predefined meaning

The language is a variation of RDFS

12. +
RDFS
Introduction by example

13. +
RDFS

W3C standard for an ontology language

RDFS introduces resources (URIs) with a predefined meaning

Inference engines that support RDFS allow to take that
meaning into account

RDFS inferences extend the RDF graph by means of inference
and hence, affect query answering

RDFS is very simple compared to SWRL or OWL, however, it
is very useful in many context, allowing for increased
productivity, easy data integration and interesting AI
applications

14. +
Building blocks

New namespace rdfs:
<http://www.w3.org/2000/01/rdf-schema#>

New categories:
Commonly,
Class names
are nouns

Classes, resources that share something in common, allow us to
group things together. For example, Employee, Company.
Resources that identify classes have rdf:type rdfs:Class

Instances, resources that are “members” of a class

15. +
Building blocks
Resources can belong to multiple classes

16. +
Building blocks (cont.)

Properties: Resources used as a predicate in statements
Commonly, Property names are
multiple words, expressing direction
and in camel-casing

17. +
RDFS Ontologies

RDFS Axioms

Are RDF triples!

RDFS ontology is an RDF graph!

An RDF graph may have a subgraph expressed in RDFS

We call the RDFS axioms/triples the Tbox of the ontology
(terminological information, predefined meaning)

The rest is the Abox of the ontology (plain data, no predefined
meaning)

18. +
Type propagation

RDFS vocabulary:
rdfs:subClassOf

Key notions

sub class (on the left)

super class (on the right)

Intuitive meaning, if :mariano is
an instance of subclass it is
also an instance of superclass

Formal meaning: subsets

Inference: type propagation
Similar to inheritance
in Object Oriented
formalisms

19. +
Type propagation

RDFS vocabulary:
rdfs:subClassOf

Key notions

sub class (on the left)

super class (on the right)

Intuitive meaning, if :mariano is
an instance of subclass it is
also an instance of superclass

Formal meaning: subsets

Inference: type propagation
Similar to inheritance
in Object Oriented
formalisms

20. +
Relation propagation

RDFS vocabulary:
rdfs:subPropertyOf

Key notions
 sub property(on the left)
 super property(on the right)

Intuitive meaning, if (x,y) are connected with subproperty
they are also connected with superproperty

Formal meaning: subsets (of binary tuples)

Inference: relationship propagation

21. +
Relation propagation

RDFS vocabulary:
rdfs:subPropertyOf

Key notions
 sub property(on the left)
 super property(on the right)

Intuitive meaning, if (x,y) are connected with subproperty
they are also connected with superproperty

Formal meaning: subsets (of binary tuples)

Inference: relationship propagation

22. +
Types by usage

RDFS vocabulary:
rdfs:domain, rdfs:range

Key notions
 domain of a triple:
the subject
 range of a triple:
the object

:p rdfs:domain :C > the domain of any
triple where :p is the predicate is an
instance of :C
(similar for rdfs:range)

Formal meaning:
if (x,y) in P, then x in :C

Inference: type assignment by property
usage

23. +
Types by usage

RDFS vocabulary:
rdfs:domain, rdfs:range

Key notions
 domain of a triple:
the subject
 range of a triple:
the object

:p rdfs:domain :C > the domain of any
triple where :p is the predicate is an
instance of :C
(similar for rdfs:range)

Formal meaning:
if (x,y) in P, then x in :C

Inference: type assignment by property
usage

24. +
Interactions

All inferences interact to allow
complex behavior

25. +
Interactions

All inferences interact to allow
complex behavior

26. +
Set intersection

Proper set intersection is not
possible in RDFS

However, expressing necessary
membership to multiple classes
is possible, i.e., A subset B AND
C
A rdfs:subClassOf B
A rdfs:subClassOf C
consider
x rdf:type A

27. +
Set intersection

Proper set intersection is not
possible in RDFS

However, expressing necessary
membership to multiple classes
is possible, i.e., A subset B AND
C
A rdfs:subClassOf B
A rdfs:subClassOf C
consider
x rdf:type A

28. +
Set intersection

Proper set intersection is not
possible in RDFS

However, expressing necessary
membership to multiple classes
is possible, i.e., A subset B AND
C
A rdfs:subClassOf B
A rdfs:subClassOf C
consider
x rdf:type A
One direction
only!

29. +
Set intersection

Similar for roles

30. +
Set intersection

Similar for roles

31. +
Set union

Proper set union is not possible
in RDFS

However, A OR B subsetOf C
B rdfs:subClassOf A
C rdfs:subClassOf A
consider
x rdf:type B
or
x rdf:type C

32. +
Set union

Proper set union is not possible
in RDFS

However, A OR B subsetOf C
B rdfs:subClassOf A
C rdfs:subClassOf A
consider
x rdf:type B
or
x rdf:type C

33. +
Set union

For roles. Aligning to a global
vocabulary

34. +
Set union

For roles. Aligning to a global
vocabulary

35. +
Equivalence

Merging vocabularies

To account for same use of
different terms (classes or
properties)

For classes or proeperties

36. +
Equivalence

Merging vocabularies

To account for same use of
different terms (classes or
properties)

For classes or proeperties

37. +
Last notes on RDFS axioms

Main new vocabulary:

rdfs:subClassOf

rdfs:subPropertyOf

rdfs:domain

rdfs:range

Different from CONSTRAINTS, missing triples are NOT a
violation

Allow to infer new information

Allows to implement system behavior!

38. +
Open lists revisited

RDFS also facilitates access to Lists

Elements of lists are a possibly
infinite set of elements of the form
rdf:_1, rdf:_2, etc
RDFS facilitates this by enforcing that:
if x rdfs:_1 y
then x rdfs:member b

Access difficult in practice

39. +
Open lists revisited

RDFS also facilitates access to Lists

Elements of lists are a possibly
infinite set of elements of the form
rdf:_1, rdf:_2, etc

Access difficult in practice
RDFS facilitates this by enforcing that:
if x rdfs:_1 y
then x rdfs:member b
More detail on this on the lecture about
RDFS semantics

40. +
Axiomatic triples

RDFS enforces certain facts to be always true

These facts are statements (triples)

Referred as Axiomatic triples

Listed in http://www.w3.org/TR/rdf-mt/
More detail on this on the lecture about
RDFS semantics

41. +
RDFS Semantic Conditions

Every resource x
x rdf:type rdfs:Resource

Every literal x
x rdf:type rdfs:Literal

… etc
More detail on this on the lecture about
RDFS semantics

42. +
Last notes on RDFS axioms

Main new vocabulary (not the only one):

rdfs:subClassOf

rdfs:subPropertyOf

rdfs:domain

rdfs:range

Different from CONSTRAINTS, missing triples are NOT a
violation

Allow to infer new information

Allows to implement system behavior!

43. +
Hands on examples
From Semantic Web for the Working Ontologist

44. +
Automatic classification of
employees (part 1)

Transform into an RDF representation

Automatically catalog objects as Employees, and as Active
employees, Suspended employees and Ex-employees
using a minimal set of “axioms”
<ID>
Project
Assignment
Absent
Until
Termination Date
22
24
-
-
34
24
Dec 23, 2012
-
73
-
-
Jun 4, 2010
Employee table. Primary key: 10
Active employees are assigned to projects

45. +
Automatic classification of
employees (part 2)

Transform into an RDF representation

Automatically catalog objects as Employees as managers
<ID>
Project Name
<Manager>
24
Project-x
34
25
Project Mayhem
22
Project table. Primary key: ID
Foreign key <Manager> to Employee table

46. + Align vocabularies
• Align corresponding properties
using RDFS
• Align with FOAF vocabulary
(when possible) using RDFS (use
foaf:name, foaf:homepage)

47. +
Annotations

48. +
Annotations

URI’s are not readable

Readable information (comments, names, etc.) can be stored
using properties, but

Property names are not standard, however, we could like some
standard names for “human oriented information”

RDFS defines:

rdfs:label
A readable name for a resource

rdfs:comment
Human focused comments
These are properties
So, subPropertyOf can be used
with them

49. +
Redirection

Redirecting to location of documents (RDF) with additional
information about a subject

No formal semantics

RDFS provides:

rdfs:seeAlso. Additional information

rdfs:definedBy. Authority information, primary source.
Recall the semantic web
idea, linked databases
These are properties
So, subPropertyOf can be used
with them