On the Semantics of R2RML and its Relationship with the Direct

MappingJuan F. Sequeda

Research in Bioinformatics and Semantic Web (RiBS) LabDepartment of Computer Science, The University of Texas at Austin

SQL query

Parser

• What are the formal semantics of R2RML?• What is the relationship between R2RML and Direct

Mapping?

Logical Query Plan

Select Logical Query Plan

Optimal Logical Query Plan

Physical Query Plan

Select Physical Query Plan

R2RML Mapping

Parser

Logical Mapping

Select Logical Mapping

Optimal Logical Mapping

Physical Mapping

Select Physical Mapping

• Exploit a well established architecture: query optimization • Organization of query optimization is applicable to

mappings

Example

Methodology

Declarative Language

Logical Perspective

Physical Perspective

Triple(S,"rdf:type”,O) ← SubjectTemplateValueTermMap(SID, T, S),

Class(SID, O)

Triple(S,P,O) ← SubjectMap(TM, SID),

PredicateObjectMap(TM, POID),

SubjectTemplateValueTermMap (SID, T, S),

PredicateConstantValueTermMap(POID, P),

ObjectColumnTermMap(POID, T, O)

Triple(S,P,O) ← SubjectMap(TM, SID),

PredicateObjectMap(TM, POID),

SubjectTemplateValueTermMap (SID, T, S),

PredicateConstantValueTermMap(POID, P),

ReferencingObjectTemplate(POID, T, O)

R2RML

Ptemplate

Pcolumn

Pconstant

Otemplate

Ocolumn

Oconstant

Stemplate

Otemplate

Ocolumn

Oconstant

Ocolumn

Oconstant

Otemplate

Ptemplate

Pcolumn

Pconstant

Otemplate

Ocolumn

Oconstant

Sconstant

Otemplate

Ocolumn

Oconstant

Ocolumn

Oconstant

Otemplate

Ptemplate

Pcolumn

Pconstant

Otemplate

Ocolumn

Oconstant

Scolumn

Otemplate

Ocolumn

Oconstant

Ocolumn

Oconstant

Otemplate

Theorem: The total number of distinct Datalog rules which can be used to generate RDF triples is 57Proof: 3 rules to generate Table triples, 27 rules to generate Local triples and 27 rules to generate Reference triples.

• Direct Mapping is the default and automatic approach of generating RDF and OWL from a Relational Database

• Semantics formalized in Datalog (see WWW2012 paper)

Hypothesis: R2RMLcore is as expressive as theDirect Mapping if views are allowed as input

Relationship between R2RML and Direct Mapping R2RMLcore

Table Triples

Local Triples

References Triples

Problem Statementempno ename dept1 Bob 1002 Alice 200

deptno dname

100 CS

200 EE

<emp/1> rdf:type ex:Employee.<emp/1> ex:name “Bob”.<emp/1> ex:name <dept/100>.<emp/2> rdf:type ex:Employee.<emp/2> ex:name “Alice”.<emp/2> ex:name <dept/200>.

<dept/100> rdf:type ex:Department.<dept/100> ex:deptName “CS”.<dept/200> rdf:type ex:Department.<dept/200> ex:deptName “EE”.

<#TriplesMap1> rr:logicalTable [ rr:tableName "emp" ]; rr:subjectMap [ rr:template "http://ex.com/emp/{empno}"; rr:class ex:Employee; ]; rr:predicateObjectMap [ rr:predicate ex:name; rr:objectMap [ rr:column "ename" ]; ]; rr:predicateObjectMap [ rr:predicate ex:department; rr:objectMap [ rr:parentTriplesMap <#TriplesMap2>; rr:joinCondition [ rr:child "dept"; rr:parent "deptno"; ]; ]; ].

<#TriplesMap2> rr:logicalTable [ rr:tableName "dept" ]; rr:subjectMap [ rr:template "http://ex.com/dept/{deptno}"; rr:class ex:Department; ]; rr:predicateObjectMap [ rr:predicate ex:deptName; rr:objectMap [ rr:column ”dname" ]; ].

What does this actually mean?

Understanding these questions can help support users and build tools.

Three types of triples: Table, Local and Reference RDF Term (S, P, O) can be generated from a Template, Constant or Column

W3C Recommendations to map Relational Databases to RDF are R2RML (customizable mapping language) and Direct Mapping (default automatic mapping) are the

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