Blank node skolemization in SPARQL without iteration - sparql

Question

Is it possible to implement blank node skolemization in SPARQL without iteration? It seems to me that iteration is required to skolemize chains of blank nodes, such as:
#prefix : <http://example.com/> .
[ a :A ;
:p1 [
a :B
]
] .
A SPARQL Update operation for skolemization can start from the blank nodes that appear as subjects only in triples without blank node objects:
DELETE {
?b1 ?outP ?outO .
?inS ?inP ?b1 .
}
INSERT {
?iri ?outP ?outO .
?inS ?inP ?iri .
}
WHERE {
{
SELECT ?b1 (uuid() AS ?iri)
WHERE {
{
SELECT DISTINCT ?b1
WHERE {
?b1 ?p1 [] .
FILTER isBlank(?b1)
FILTER NOT EXISTS {
?b1 ?p2 ?b2 .
FILTER isBlank(?b2)
}
}
}
}
}
?b1 ?outP ?outO .
OPTIONAL {
?inS ?inP ?b1 .
}
}
This operation can be repeated until no blank nodes are found in the data:
ASK {
?bnode ?p [] .
FILTER isBlank(?bnode)
}
Is it possible to avoid the iteration and implement the blank node skolemization in a single SPARQL Update operation?
(Also, this approach assumes there are no "orphan" blank nodes (i.e. blank nodes that appear only as objects).)

Answer 1

I found a two-step solution skolemising subjects and objects separately and storing the blank node aliases (links between blank nodes and IRIs via owl:sameAs) as intermediate data:
PREFIX : <http://example.com/>
PREFIX owl: <http://www.w3.org/2002/07/owl#>
####################
# Rewrite subjects #
####################
DELETE {
?bnode ?p ?o .
}
INSERT {
?iri ?p ?o .
GRAPH :aliases {
?bnode owl:sameAs ?iri .
}
}
WHERE {
{
SELECT ?bnode (uuid() AS ?iri)
WHERE {
{
SELECT DISTINCT ?bnode
WHERE {
?bnode ?p [] .
FILTER isBlank(?bnode)
}
}
}
}
?bnode ?p ?o .
}
;
###################
# Rewrite objects #
###################
DELETE {
?s ?p ?bnode .
}
INSERT {
?s ?p ?iri .
}
WHERE {
{
SELECT ?bnode ?iri
WHERE {
{
SELECT DISTINCT ?bnode
WHERE {
[] ?p ?bnode .
FILTER isBlank(?bnode)
}
}
OPTIONAL {
GRAPH :aliases {
?bnode owl:sameAs ?_iri .
}
}
BIND (coalesce(?_iri, uuid()) AS ?iri)
}
}
?s ?p ?bnode .
}
;
############################
# Clear blank node aliases #
############################
CLEAR GRAPH :aliases