SKOS Thesaurus Management in PostgreSQL

This cookbook chapter shows how to load, query, and validate a SKOS taxonomy inside pg_ripple using the W3C-conformant SKOS entailment rules added in v0.98.0.

What is SKOS?

SKOS (Simple Knowledge Organization System) is the W3C standard vocabulary for thesauri, taxonomies, classification schemes, and subject heading systems. A SKOS taxonomy consists of skos:Concept nodes linked by skos:broader/skos:narrower hierarchies, skos:related associative links, and labelled with skos:prefLabel.

Loading the SKOS Rule Set

-- Load all 28 W3C SKOS entailment rules (S7–S45).
SELECT pg_ripple.load_builtin_rules('skos');

-- Or use the named bundle API for versioned activation:
SELECT pg_ripple.load_datalog_bundle('skos');

Once loaded, pg_ripple automatically infers:

• skos:broaderTransitive and skos:narrowerTransitive transitive closures
• Inverse skos:narrower/skos:broader pairs
• skos:related symmetry
• Sub-property hierarchies for mapping predicates (broadMatch, exactMatch, etc.)
• Label sub-properties (prefLabel → rdfs:label)

Loading a Thesaurus

-- Load Turtle-encoded thesaurus data.
SELECT pg_ripple.load_turtle($$
@prefix skos: <http://www.w3.org/2004/02/skos/core#> .
@prefix ex:   <http://example.org/thesaurus/> .

ex:Animals a skos:ConceptScheme ;
    skos:hasTopConcept ex:Mammals .

ex:Mammals a skos:Concept ;
    skos:prefLabel "Mammals"@en ;
    skos:inScheme ex:Animals .

ex:Cats a skos:Concept ;
    skos:prefLabel "Cats"@en ;
    skos:broader ex:Mammals .

ex:Dogs a skos:Concept ;
    skos:prefLabel "Dogs"@en ;
    skos:broader ex:Mammals .

ex:Cats skos:related ex:Dogs .
$$);

-- Run inference to materialise transitive closures.
SELECT pg_ripple.infer('skos');

Querying Hierarchies with SPARQL

-- Find all broader transitive ancestors of Cats.
SELECT *
FROM pg_ripple.sparql($$
    SELECT ?ancestor WHERE {
        <http://example.org/thesaurus/Cats>
            <http://www.w3.org/2004/02/skos/core#broaderTransitive> ?ancestor .
    }
$$);

-- Find all concepts in the Mammals subtree (narrowerTransitive closure).
SELECT *
FROM pg_ripple.sparql($$
    SELECT ?child WHERE {
        <http://example.org/thesaurus/Mammals>
            <http://www.w3.org/2004/02/skos/core#narrowerTransitive> ?child .
    }
$$);

-- Cross-scheme mapping: find exactMatch counterparts.
SELECT *
FROM pg_ripple.sparql($$
    SELECT ?concept ?match WHERE {
        ?concept <http://www.w3.org/2004/02/skos/core#exactMatch> ?match .
    }
$$);

SQL Helper Functions

The five SQL helper functions provide convenient access to common thesaurus operations without writing SPARQL:

-- Traverse the broaderTransitive closure.
SELECT * FROM pg_ripple.skos_ancestors(
    'http://example.org/thesaurus/Cats'
);
-- Returns: (ancestor_iri, depth)

-- Traverse the narrowerTransitive closure.
SELECT * FROM pg_ripple.skos_descendants(
    'http://example.org/thesaurus/Mammals'
);
-- Returns: (descendant_iri, depth)

-- Look up the preferred label in English.
SELECT pg_ripple.skos_label(
    'http://example.org/thesaurus/Cats',
    'en'
);
-- Returns: "Cats"

-- Find all related concepts.
SELECT * FROM pg_ripple.skos_related(
    'http://example.org/thesaurus/Cats'
);
-- Returns: (related_iri, relation) — e.g. Dogs, skos:related

-- Find sibling concepts (sharing a common skos:broader parent).
SELECT * FROM pg_ripple.skos_siblings(
    'http://example.org/thesaurus/Cats'
);
-- Returns: (sibling_iri, shared_broader_iri) — e.g. Dogs, Mammals

Enabling SKOS-XL Extended Labels

SKOS-XL allows attaching structured metadata to labels. pg_ripple's SKOS-XL rule set automatically projects skosxl:Label instances to plain skos:prefLabel triples:

SELECT pg_ripple.load_builtin_rules('skosxl');
SELECT pg_ripple.infer('skosxl');

After loading, a triple <concept> skosxl:prefLabel <label> with <label> skosxl:literalForm "text"@en will produce <concept> skos:prefLabel "text"@en automatically.

Running Integrity Checks

The "skos-integrity" shape bundle validates 10 structural integrity conditions from the W3C SKOS specification:

-- Activate the integrity shape bundle (automatically loads skos-transitive).
SELECT pg_ripple.load_shape_bundle('skos-integrity');

-- Run validation.
SELECT * FROM pg_ripple.validate_skos();
-- Returns: (violation_id, subject, message)
-- Example violation: SKOS-IC-05: multiple prefLabels with same language tag.

The 10 validators cover:

Named Bundle API

Use load_datalog_bundle instead of load_builtin_rules when you need versioned, machine-checkable bundle tracking:

SELECT pg_ripple.load_datalog_bundle('skos');
SELECT pg_ripple.load_datalog_bundle('skos-transitive');

-- Check what is active.
SELECT * FROM pg_ripple.active_datalog_bundles;
-- Returns: (bundle_name, bundle_version, loaded_at, named_graph)

Performance Notes

• skos:broaderTransitive closure materialisation via WITH RECURSIVE … CYCLE takes ~200 ms for 1,000 concepts and ~8 s for 100,000 concepts on first materialisation.
• Subsequent IVM updates (when enabled) take ~10 ms per new skos:broader triple.
• skos_ancestors() runs a live WITH RECURSIVE query in ~5 ms for depth ≤ 10, using the materialised broaderTransitive triples as the base.

See Also

• SKOS Reference — W3C Recommendation
• Datalog Reference — built-in RDFS and OWL RL rule support
• SQL Functions Reference — all pg_ripple SQL functions