Ontop

# First data source: university 1

As a first step, we focus on the database of a first university. It has the schema uni1. It is composed of 5 tables.

# uni1.student

The table uni1.student contains the local ID, first and last names of the students.

s_id first_name last_name
1 Mary Smith
2 John Doe

The column s_id is a primary key.

# uni1.academic

Similarly, the table uni1.academic contains the local ID, first and last names of the academic staff, but also information about their position.

a_id first_name last_name position
1 Anna Chambers 1
2 Edward May 9
3 Rachel Ward 8

The column position is populated with magic numbers:

  • 1 -> Full Professor
  • 2 -> Associate Professor
  • 3 -> Assistant Professor
  • 8 -> External Teacher
  • 9 -> PostDoc

The column a_id is a primary key.

# uni1.course

The table uni1.course contains the local ID and the title of the courses.

c_id title
1234 Linear Algebra

The column c_id is a primary key.

# uni1.teaching

The table uni1.teaching contains the n-n relation between courses and teachers.

c_id a_id
1234 1
1234 2

There is no primary key, but two foreign keys to the tables uni1.course and uni1.academic.

# uni1.course-registration

The table uni1.course-registration contains the n-n relation between courses and students.

c_id s_id
1234 1
1234 2

There is no primary key, but two foreign keys to the tables uni1.course and uni1.student.

# Ontology: classes and properties

  1. Download this OWL ontology file.
  2. Download this mapping file.
  3. Download this properties file.
  4. In Protégé, go to File/Open... to load the ontology file (be sure you have all three files in the same folder).
  5. In the tab Classes you can visualize the class hierarchy
  6. In the tab Object properties you can see the properties attends, isGivenAt, isSupervisedBy, isTaughtBy and teaches (with its two sub-properties givesLab and givesLecture).
  7. In the tab Data properties you can see the properties firstName, lastName and title.

# Mappings

  1. Go to the Window -> Tabs -> Ontop mapping tab
  2. Test the already defined connection configuration using the Test Connection button
  3. Switch to the Mapping Manager tab in the ontop mappings tab
  4. You should see a first mapping assertion called uni1-student
  5. Double-click on it to observe it and then close this pop-up window.

# Mapping uni1-student

  • Target:
:uni1/student/{s_id} a :Student ;
    foaf:firstName {first_name}^^xsd:string ;
    foaf:lastName {last_name}^^xsd:string .
  • Source:
SELECT *
FROM "uni1"."student"

Some remarks:

  • The target part is described using a Turtle-like syntax (opens new window) while the source part is a regular SQL query.
  • We used the primary key s_id to create the IRI. As we will see later, this practice enables Ontop to remove self-joins, which is very important for optimizing the query performance.
  • This entry could be split into three mapping assertions
:uni1/student/{s_id} a :Student .
:uni1/student/{s_id} foaf:firstName {first_name}^^xsd:string .
:uni1/student/{s_id} foaf:lastName {last_name}^^xsd:string .

Let us now add the other mapping assertions by clicking on create:

# Mapping uni1-academic

  • Target:
:uni1/academic/{a_id} a :FacultyMember ;
    foaf:firstName {first_name}^^xsd:string ;
    foaf:lastName {last_name}^^xsd:string .
  • Source:
SELECT *
FROM "uni1"."academic"

# Mapping uni1-course

  • Target:
:uni1/course/{c_id} a :Course ;
    :title {title} ;
    :isGivenAt :uni1/university .
  • Source:
SELECT *
FROM "uni1"."course"

# Mapping uni1-teaching

  • Target:
:uni1/academic/{a_id} :teaches :uni1/course/{c_id} .
  • Source:
SELECT *
FROM "uni1"."teaching"

# Mapping uni1-registration

  • Target:
:uni1/student/{s_id} :attends :uni1/course/{c_id} .
  • Source:
SELECT *
FROM "uni1"."course-registration"

# Mapping uni1-fullProfessor

  • Target:
:uni1/academic/{a_id} a :FullProfessor .
  • Source:
SELECT *
FROM "uni1"."academic"
WHERE "position" = 1

Then proceed in a similar way for the other positions (associate professor, assistant professor, external teacher, postdoc).

# SPARQL

  1. Run Protégé and go to the Window -> Tabs -> Ontop SPARQL tab
  2. Select Ontop in the Reasoner menu
  3. Start the reasoner
  4. Add a query in the Query Manager and run the following query in SPARQL query editor:
PREFIX : <http://example.org/voc#>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT DISTINCT ?prof ?lastName {
  ?prof a :FullProfessor ; foaf:lastName ?lastName .
}

Tip: do a right click on the SPARQL query field to visualize the generated SQL query.

# Inference

Ontop embeds some inference capabilities and is thus capable of answering a query like the following:

PREFIX : <http://example.org/voc#>
SELECT DISTINCT ?teacher {
  ?teacher a :Teacher .
}

These inference capabilities can be, for a large part, understood as the ability to infer new mappings from the original mappings and the ontological axioms (e.g. Professor is a sub-class of Teacher). We will discuss it later in this tutorial.

← Second data source: university 2 →