ConceptDBISAI-written · not yet proofread

Reification

Also Mediator Entity · Statement Node

Reification expresses an n-ary fact (n > 2) with binary RDF triples by introducing a mediator entity (in Wikidata a statement node) that the subject points to and that points to every further component of the fact.

Know these firstRDF Triple

Properties#

Why it is needed. Splitting an $n$ -ary fact into independent binary triples from the subject loses which components belong together (see the example).
Relational view. In a relational database the same fact is simply one row of a table with $n$ columns; reification is the price RDF pays for having only triples.
One mediator per fact. Two facts with the same predicate (two awards, two terms of office) need two different mediators; otherwise their components mix again.
Queries become harder. A query must walk subject → mediator → component, i.e. use more triples and the right predicate variants; a shortcut triple (wdt:) that skips the mediator cannot reach the extra components.

Example#

Meryl Streep’s Oscars:

Film	Year	Award
Kramer vs. Kramer	1980	Best Supporting Actress
Sophie’s Choice	1982	Best Actress
The Iron Lady	2011	Best Actress

Naive triples lose information

<Meryl_Streep> <won_award>          <Best_Actress>
<Meryl_Streep> <won_award_in_year>  "1982"
<Meryl_Streep> <won_award_in_year>  "2011"
<Meryl_Streep> <won_award_for_film> <Sophies_Choice>
<Meryl_Streep> <won_award_for_film> <The_Iron_Lady>
...

It is no longer recoverable which award was won for which film in which year.

With a mediator (Wikidata statement node) per award:

wd:Q873           p:P166    wds:Q873-6ad4311a .  # Meryl Streep → statement node
wds:Q873-6ad4311a ps:P166   wd:Q103618 .         # which award (Best Actress)
wds:Q873-6ad4311a pq:P585   "2011" .             # point in time
wds:Q873-6ad4311a pq:P1686  wd:Q269810 .         # for work (The Iron Lady)

Grover Cleveland held the office of US president twice (from 1885 and from 1893), so he gets two statement nodes (exam WS 23/24 retake, Task 3.2):

wd:Q35171    p:P39   wds:Q35171-1 .
wds:Q35171-1 ps:P39  wd:Q11696 .     # President of the United States
wds:Q35171-1 pq:P580 "1885" .        # start time
wd:Q35171    p:P39   wds:Q35171-2 .
wds:Q35171-2 ps:P39  wd:Q11696 .
wds:Q35171-2 pq:P580 "1893" .

Sources#

Slides 07, pp. 19–20 (n-ary relations), p. 21 (mediator entities)
DBIS_VL07 > Reification, DBIS_VL07 > Mediator entities (statement nodes)
Exams: DBIS Exam WS2324-2 (not published) (Task 3.2)

Connections

Prerequisites know first1

RDF Triple

Mentioned in inline links6

RDF Triple
Only binary relations. A triple relates exactly two things. Facts with more components ( $n$ -ary relations, $n > 2$ , e.g. award + year + film) need Reification. §
SPARQL Query Structure
For n-ary facts in Wikidata (e.g. an award and the film it was won for), the query has to go through statement nodes with the Wikidata Prefix System (see Reification). §
Wikidata Prefix System
Wikidata stores every claim about an entity both as a direct triple and via a statement node (a mediator, see Reification). A property ID Pxxx therefore appears with different IRI prefixes: §
VL07 – Knowledge Graphs, SPARQL, SPARQL to SQL
Reification — modelling $n$ -ary facts with mediator entities (statement nodes) §
SPARQL and Knowledge Graphs
Reification — expressing $n$ -ary facts with mediator entities (statement nodes) §
Binary only → reification. Triples are binary, so a fact like “award, year, film” needs a mediator (Reification). Wikidata does this with statement nodes and offers two views of each property: the shortcut wdt: (no mediator, no qualifiers)… §
VL01 – Overview, RDF & SPARQL, more SPARQLADIS
A quick recap of DBIS Lecture 7: the RDF data model, basic SPARQL and Reification. If it goes too fast, rewatch DBIS Lecture 7 before working on Exercise Sheet 1 (Slide 7). §

Search in these mentions →