MarkLogic 10 Product Documentation

Semantic Graph Developer's Guide — Chapter 10

XQuery Library Module for Semantics

Some of the Semantics XQuery functions are built-in functions that do not require an import statement, while others are implemented in an XQuery library module that requires an import statement. To simplify things, MarkLogic recommends that you import the Semantics API library into every XQuery module or JavaScript module that uses the Semantics API.

import module namespace sem = "http://marklogic.com/semantics" 
  at "/MarkLogic/semantics.xqy";

var sem = require("/marklogic/semantics.xqy");

Generating Triples

The XQuery sem:rdf-builder function is a powerful tool for dynamically generating triples in the Semantics API. (For JavaScript, the function is sem.rdfBuilder.)

The function builds triples from the CURIE and blank node syntaxes. Blank nodes specified with a leading underscore ( _ ) are assigned blank node identifiers, and maintain state across multiple invocations; for example, "_:person1" refers to the same node as a later invocation that also mentions "_:person1". For example:

xquery version "1.0-ml";
import module namespace sem = "http://marklogic.com/semantics" 
  at "/MarkLogic/semantics.xqy";
 
 let $builder := sem:rdf-builder((), sem:iri("my-named-graph"))
 let $t1 := $builder("_:person1", "a", "foaf:Person") 
 let $t2 := $builder("_:person2", "a", "foaf:Person") 
 let $t3 := $builder("_:person1", "foaf:knows", "_:person2") 
 return ($t1,$t2,$t3) 

=>

@prefix xs: <http://www.w3.org/2001/XMLSchema#> .

<http://marklogic.com/semantics/blank/4892021155019117627>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type/>
<http://xmlns.com/foaf/0.1/Person> .

<http://marklogic.com/semantics/blank/6695700652332466909>
        <http://xmlns.com/foaf/0.1/knows> 
        _:bnode4892021155019117627 ;
        <http://www.w3.org/1999/02/22-rdf-syntax-ns#type/>
        <http://xmlns.com/foaf/0.1/Person> .

In the example, there are three triples generated in Turtle serialization using sem:rdf-builder. The triples represent the following facts; that person1 and person2 are people, and that their relationship is that person1 knows person2.

Note the following:

• The first parameter accepts an optional set of prefix mappings, which in this example is an empty argument. Since empty means default, the $common-prefixes are used for the first argument. The second argument is a named graph for the sem:rdf-builder output.
• In the predicate position, the special value of "a" is expanded to the full IRI for rdf:type.
• Human-readable CURIEs for common prefixes are used, such as foaf:knows instead of long IRIs. See Working with CURIEs.
• The blank nodes produced in the third triple match the identity of those defined in the first and second.

Extracting Triples from Content

With the sem:rdf-builder function you can easily extract triples from existing content or the results of a SPARQL query and quickly construct RDF graphs for querying or inserting into your database.

Assume that you have a web page that lists cities and countries that are sorted and ranked by the cost of living (COL), which is based on a Consumer Priced Index (CPI) and CPI-based inflation rate. The inflation rate is defined as the annual percent change in consumer prices compared with the previous year's consumer prices. Using a reference point of Monterrey, Mexico with an assigned a value of 100, the Inflation value of every other city in the database is calculated by comparing their COL to that of Monterrey. For example, an Inflation value of 150, means that the COL is 50% more expensive than living in Monterrey.

These values are fictional and are not based on any official sources.

The underlying HTML code for the COL table may resemble the following:

<table class="city-index"
style="max-width:58%;float:left;margin-right:2em;">
    <thead>
        <tr>
            <th>Ranking</th>
            <th class="city-name">City</th>
            <th class="inflation">Inflation 
            <a href="#inflation-explanation">*</a></th>
        </tr>
    </thead>
          
           <tbody><tr>
              <td class="ranking">1</td>
              <td class="city-name">
              <a href="http://www.example.org/IncreasedCoL/london">
              London (United Kingdom)</a></td>
              <td class="inflation">270</td>
            </tr>
          
            <tr>
              <td class="ranking">2</td>
              <td class="city-name">
              <a href="http://www.example.org/IncreasedCoL/stockholm">
              Stockholm (Sweden)</a></td>
              <td class="inflation">266</td>
            </tr>
          
            <tr>
              <td class="ranking">3</td>
              <td class="city-name">
              <a href="http://www.example.org/IncreasedCoL/zurich">
              Zurich (Switzerland)</a></td>
              <td class="inflation">251</td>
            </tr>
          
            <tr>
              <td class="ranking">4</td>
              <td class="city-name">
              <a href="http://www.example.org/IncreasedCoL/geneva">
              Geneva (Switzerland)</a></td>
              <td class="inflation">247</td>
            </tr>
          
            <tr>
              <td class="ranking">5</td>
              <td class="city-name">
              <ahref="http://www.example.org/IncreasedCoL/new-york">
              New York City (United States)</a></td>
              <td class="inflation">225</td>
            </tr>

This example uses the sem:rdf-builder function to extract triples from the HTML content. The function takes advantage of the fact that the HTML code is already well-formed and has a useful classification node (@class):

import module namespace sem = "http://marklogic.com/semantics" 
  at "/MarkLogic/semantics.xqy";
declare namespace html="http://www.w3.org/1999/xhtml";

let $doc := xdmp:tidy(xdmp:document-get("C:\Temp\CoLIndex.html",
      <options xmlns="xdmp:document-get">
        <repair>none</repair>
        <format>text</format>
      </options>))[2]

let $rows := ($doc//html:tr)[html:td/@class eq 'ranking']
let $builder := sem:rdf-builder
                 (sem:prefixes("my: http://example.org/vocab/"))
for $row in $rows
let $bnode-name := "_:" || $row/html:td[@class eq 'ranking']
return (
        $builder($bnode-name, "my:rank", xs:decimal(
                 $row/html:td[@class eq 'ranking'] )),
        $builder($bnode-name, "rdfs:label", xs:string(
                 $row/html:td[@class eq 'city-name'] )),
        $builder($bnode-name, "my:coli", xs:decimal(
                 $row/html:td[@class eq 'inflation'] ))
        )

The results are returned as in-memory triples:

Parsing Triples

This example extends the previous example and inserts parsed triples into the database by using the sem:rdf-insert function:

import module namespace sem = "http://marklogic.com/semantics" 
  at "/MarkLogic/semantics.xqy";
declare namespace html="http://www.w3.org/1999/xhtml";

let $doc := xdmp:tidy(xdmp:document-get("C:\Temp\CoLIndex.html",
      <options xmlns="xdmp:document-get">
        <repair>none</repair>
        <format>text</format>
      </options>))[2]

let $rows := ($doc//html:tr)[html:td/@class eq 'ranking']
let $builder := sem:rdf-builder(
                  sem:prefixes("my: http://example.org/vocab/"))
for $row in $rows
let $bnode-name := "_:" || $row/html:td[@class eq 'ranking']
let $triples := $row
return (sem:rdf-insert(( 
      $builder($bnode-name, "my:rank", xs:decimal
            ( $row/html:td[@class eq 'ranking'] )),
      $builder($bnode-name, "rdfs:label", xs:string
            ( $row/html:td[@class eq 'city-name'] )),
      $builder($bnode-name, "my:coli", xs:decimal
            ( $row/html:td[@class eq 'inflation'] )))
))

The document IRIs are returned as strings:

For more information about inserting and parsing triples with XQuery, see Loading Triples with XQuery.

The parser ensures well-formed markup as the triples are inserted as schema-valid triples and indexed with the Triples index, provided it is enabled. See Enabling the Triple Index.

Use fn:doc to view the contents of the documents and verify the triples.

fn:doc("/triplestore/2ca88f284b7dd96d.xml")

One document is created for each blank node identifier ($bnode-name).

During the generation process $builder maintains state eliminating the need to keep track of every blank node label and ensuring that they map to the same sem:blank value.

The Semantics API includes a repair option for the N-Quad and Turtle parsers. During a normal operation, the RDF parsers perform these tasks:

• Turtle parsing uses the base IRI to resolve relative IRIs. If the result is relative, an error is raised.
• N-Quad parsing does not resolve using the base IRI. If a IRI in the document is relative, an error is raised.

During a repair operation the RDF parsers perform this task:

• Turtle parsing uses the base IRI to resolve relative IRIs. No error is raised for resultant relative IRIs.
• N-Quad parsing also uses the base IRI to resolve relative IRIs. No error is raised for resultant relative IRIs.

Exploring Data

The Semantics API provides functions to access RDF data in a database. This section focuses on the following topics:

• sem:triple Functions
• Transitive Closure

sem:triple(sem:iri("http://id.loc.gov/authorities/subjects/
sh85040989"), 
sem:curie-expand("skos:prefLabel"), 
rdf:langString("Education", "en"))

=>
@prefix xs: <http://www.w3.org/2001/XMLSchema#> .

<http://id.loc.gov/authorities/subjects/sh85040989>
<http://www.w3.org/2004/02/skos/core#prefLabel/> 
"Education"@en .

http://www.w3.org/TR/skos-reference/

sem:transitive-closure(
   $seeds as sem:iri*,
   $predicates as sem:iri*,
   $limit as xs:integer
) as sem:iri*

<http://id.loc.gov/authorities/subjects/sh85002310/>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type/>
<http://www.w3.org/2004/02/skos/core#Concept/> . 

<http://id.loc.gov/authorities/subjects/sh85002310/>
<http://www.w3.org/2004/02/skos/core#prefLabel/> 
"Agricultural education"@en . 

<http://id.loc.gov/authorities/subjects/sh85002310/>
<http://www.w3.org/2008/05/skos-xl#altLabel/>
_:bnode7authoritiessubjectssh85002310 . 

_:bnode7authoritiessubjectssh85002310
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type/>
<http://www.w3.org/2008/05/skos-xl#Label/> . 

_:bnode7authoritiessubjectssh85002310
<http://www.w3.org/2008/05/skos-xl#literalForm/> 
"Education, Agricultural"@en . 

<http://id.loc.gov/authorities/subjects/sh85002310/>
<http://www.w3.org/2004/02/skos/core#broader/>
<http://id.loc.gov/authorities/subjects/sh85133121/> . 

<http://id.loc.gov/authorities/subjects/sh85002310/>
<http://www.w3.org/2004/02/skos/core#narrower/>
<http://id.loc.gov/authorities/subjects/sh85118332/> . 

<http://id.loc.gov/authorities/subjects/sh85133121/>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type/>
<http://www.w3.org/2004/02/skos/core#Concept/> . 
<http://id.loc.gov/authorities/subjects/sh85133121/>
<http://www.w3.org/2004/02/skos/core#prefLabel/> 
"Technical education"@en . 

<http://id.loc.gov/authorities/subjects/sh85002310/>
<http://www.w3.org/2004/02/skos/core#broader/>
<http://id.loc.gov/authorities/subjects/sh85133121/> . 

xquery version "1.0-ml";
import module namespace sem = "http://marklogic.com/semantics" 
  at "/MarkLogic/semantics.xqy";

let $triple-subject := sem:triple-subject(cts:triples((), sem:curie-expand("skos:prefLabel"), 
rdf:langString("Agricultural education", "en")))
return 
sem:transitive-closure($triple-subject, sem:curie-expand("skos:broader"), 3)

=>
<http://id.loc.gov/authorities/subjects/sh85133121/>
<http://id.loc.gov/authorities/subjects/sh85002310/>
<http://id.loc.gov/authorities/subjects/sh85026423/>
<http://id.loc.gov/authorities/subjects/sh85040989/>

<http://id.loc.gov/authorities/subjects/sh85040989/>
<http://www.w3.org/2004/02/skos/core#prefLabel/> 
"Education"@en . 
<http://id.loc.gov/authorities/subjects/sh85040989/>
<http://www.w3.org/2004/02/skos/core#broader/>
<http://id.loc.gov/authorities/subjects/sh85026423/> . 
...
<http://id.loc.gov/authorities/subjects/sh85026423/>
<http://www.w3.org/2004/02/skos/core#prefLabel/> 
"Civilization"@en . 
...