Alternatives to the OPTIONAL fallback SPARQL pattern? -


i need retrieve single image each resource in target set, testing multiple non-exclusive predicates priority order.

i'm currently using standard optional fallback pattern along lines of

select ?r ?i {      ?r dbo:automobile .      optional { ?r <http://dbpedia.org/ontology/thumbnail> ?i }     optional { ?r <http://xmlns.com/foaf/0.1/depiction> ?i }     optional { ?r <http://xmlns.com/foaf/0.1/logo> ?i }     optional { ?r <http://schema.org/image> ?i }  }

but approach turning out troublesome on backends: aware of simple/efficient alternative?

what's problem optionals? repeated use of ?i?

a different approach each alternative , pick first 1 set.

select ?r ?i {      ?r dbo:automobile .     optional { ?r <http://dbpedia.org/ontology/thumbnail> ?i1 }     optional { ?r <http://xmlns.com/foaf/0.1/depiction> ?i2 }     optional { ?r <http://xmlns.com/foaf/0.1/logo> ?i3 }     optional { ?r <http://schema.org/image> ?i4 }     bind(coalesce(?i1,?i2,?i3,?i4) ?i) }

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