Dr. Alexandra I. Cristea CS 253: Topics in Database Systems: C4

Previously we looked at: –XML, and its query language(s) –RDF Next: –RDF query languages

RDF query languages

Proposals SPARQL – RDQL – RQL – SeRQL – Triple: – N3: – Comparison of languages: –

SeRQL

Introduction SeRQL "Sesame RDF Query Language", pronounced "circle new RDF/RDFS query language currently being developed by Aduna as part of Sesame. It combines (best?) features of other (query) languages (RQL, RDQL, N-Triples, N3) and adds some of its own.

Sesame open source RDF framework with support for RDF Schema inferencing and querying. Originally, it was developed by Aduna (then known as Aidministrator) as a research prototype for the EU research project On-To- Knowledge.AdunaOn-To- Knowledge further developed and maintained by Aduna in cooperation with NLnet Foundation, developers from Ontotext, and a number of volunteer developersNLnet FoundationOntotext

SeRQL's features Graph transformation. RDF Schema support. XML Schema datatype support. Expressive path expression syntax. Optional path matching.

SeRQL basic building blocks RDF: –URIs, –literals and –variables URIs and literals variables

Variables identified by names. must start with a letter or an underscore ('_') and can be followed by zero or more letters, numbers, underscores, dashes ('-') or dots ('.'). Examples: Var1 _var2 unwise.var-name_isnt-it SeRQL keywords are not allowed to be used as variable names.

(reserved) Keywords Currently: select, construct, from, where, using, namespace, true, false, not, and, or, like, label, lang, datatype, null, isresource, isliteral, sort, in, union, intersect, minus, exists, forall, distinct, limit, offset. case-insensitive, (unlike variable names).

URIs full URIs abbreviated URIs (QNames)

Full URIs must be surrounded with " ". Tend to be long (!!) Examples:

Abbreviated URIs (QNames) Components: defined prefix (for the namespace) and a colon ( : ), then the URI part that is not a namespace Examples: sesame:index.html rdf:type foaf:Person

Literals Parts: –label, –language tag, and –datatype Examples: "foo" " "^^ " "^^rdf:XMLLiteral Optional, mutually exclusive Language tag datatype label

Blank nodes RDF has a notion of blank nodes. –nodes that are not labelled with a URI or literal. –Interpretation ( ): "there exists a node such that..." –Blank nodes have internal identifiers Shortcut in SeRQL: _:bnode1 Attention: problem of non-portability!!!

Path expressions expressions that match specific paths through an RDF graph usually, triples = path expressions of length 1 in SeRQL: arbitrary length

Basic path expressions Query: –persons who work for (companies that are) IT companies.

Original (possible) RDF: rdf:resource= rdf:resource= chema#ITCompany

Basic path expressions Query: –persons who work for (companies that are) IT companies. {Person} foo:worksFor {Company} rdf:type {foo:ITCompany} Person Company Triple (length =1):

Multiple Path Expressions Separated with commas Example: {Person} ex:worksFor {Company}, {Company} rdf:type {ex:ITCompany}

Non-interesting nodes Can be left empty Examples: {Person} ex:worksFor {} rdf:type {ex:ITCompany} {Painting} ex:painted_by {} ex:name {"Picasso"}

Path expression short cuts Multi-value nodes Branches Reified statements

Multi-valued nodes Multiple objects: {subj1} pred1 {obj1, obj2, obj3} Multiple subjects: {subj1, subj2, subj3} pred1 {obj1} Condition: disjoint !!

Branches {subj1} pred1 {obj1}; pred2 {obj2} Equivalent to: {subj1} pred1 {obj1}, {subj1} pred2 {obj2}

Reified statements { {reifSubj} reifPred {reifObj} } pred {obj} Equivalent to: {_Statement} rdf:type {rdf:Statement}, {_Statement} rdf:subject {reifSubj}, {_Statement} rdf:predicate {reifPred}, {_Statement} rdf:object {reifObj}, {_Statement} pred {obj}

Optional Path Expressions {Person} ex:name {Name}; ex:age {Age}; [ex: { Address}]

Queries in SeRQL Select queries: –returning a table of values, or a set of variable-value bindings. –SELECT, FROM, WHERE, LIMIT, OFFSET and USING NAMESPACE Construct queries: –returns a true RDF graph –CONSTRUCT, FROM, WHERE, LIMIT, OFFSET and USING NAMESPACE

Select queries SELECT C FROM {C} rdf:type {rdfs:Class} –returns all URIs of classes SELECT DISTINCT * FROM {Country1} ex:borders {} ex:borders {Country2} USING NAMESPACE ex =

Construct queries CONSTRUCT {Parent} ex:hasChild {Child} FROM {Child} ex:hasParent {Parent} USING NAMESPACE ex = CONSTRUCT * FROM {SUB} rdfs:subClassOf {SUPER} –This query extracts all rdfs:subClassOf relations from an RDF graph.

WHERE clause Optional; Specifies Boolean constraints SELECT Country FROM {Country} ex:population {Population} WHERE Population < " "^^xsd:positiveInteger USING NAMESPACE ex =

Nested WHERE clauses Query 1 (normal WHERE-clause): SELECT Name, Address FROM {Person} foaf:name {Name}; [ex: { Address}] WHERE Address LIKE "g*" Query 2 (nested WHERE-clause): SELECT Name, Address FROM {Person} foaf:name {Name}; [ex: { Address} WHERE Address LIKE "g*"] at most one nested WHERE-clause per optional path expression, and at most one 'normal' WHERE-clause

Results WHERE queries Query 1 Name Address Query 2 (nested WHERE) Name Address Michael Rubens

LIKE operator SELECT Country FROM {Country} ex:name {Name} WHERE Name LIKE netherlands" IGNORE CASE USING NAMESPACE ex =

Built-in predicates {X} serql:directSubClassOf {Y} {X} serql:directSubPropertyOf {Y} {X} serql:directType {Y}

Set combinatory operations Union Intersect Minus

Union SELECT title FROM {book} dc10:title {title} UNION SELECT title FROM {book} dc11:title {title} USING NAMESPACE dc10 =, dc11 =

Intersect SELECT creator FROM {album} dc10:creator {creator} INTERSECT SELECT creator FROM {album} dc11:creator {creator} USING NAMESPACE dc10 =, dc11 =

Minus (difference) SELECT title FROM {album} dc10:title {title} MINUS SELECT title FROM {album} dc10:title {title}; dc10:creator {creator} WHERE creator like "Paul" USING NAMESPACE dc10 =, dc11 =

NULL values SELECT * FROM {X} Y {Z} WHERE isLiteral(Z) AND datatype(L) = NULL –to check that a literal doesn't have a datatype;

Query Nesting IN ANY, ALL EXISTS

IN SELECT name FROM {} rdf:type {ex:Person}; ex:name {name} WHERE name IN ( SELECT n FROM {} rdf:type {ex:Author}; ex:name {n} ) USING NAMESPACE ex = retrieve all names of Persons, but only those names that also appear as names of Authors.

ANY, ALL SELECT highestValue FROM {node} ex:value {highestValue} WHERE highestValue >= ALL ( SELECT value FROM {} ex:value {value} ) USING NAMESPACE ex =

EXISTS SELECT name, hobby FROM {} rdf:type {ex:Person}; ex:name {name}; ex:hobby {hobby} WHERE EXISTS ( SELECT n FROM {} rdf:type {ex:Author}; ex:name {n}; ex:authorOf {} WHERE n = name ) USING NAMESPACE ex =

RDF Query Languages Conclusion We have learned: –There is a high competition for providing The RDF query language –No standards as yet –We have looked in more details at one of them, SeRQL, as it is an implementers language paired with an existing RDF repository tool, Sesame –Many features in SeRQL remind us of SQL, thus learning threshold should be low

Next: –OWL