KnarQL: handling of statements in OPTIONAL and UNION

[tobiasschweizer]

The current design expects all statements to be contained in the results (one or more matches).

The use of OPTIONAL and UNION, however, introduces statements that may match. This means that cannot be expected to be present in the results.

Consider the following SPARQL query with a UNION:

 SELECT DISTINCT ?letter (GROUP_CONCAT(?person1; SEPARATOR='') AS ?person1Concat) (GROUP_CONCAT(?date; SEPARATOR='') AS ?dateConcat) (GROUP_CONCAT(?person1__LinkValue; SEPARATOR='') AS ?person1__LinkValueConcat)
 WHERE {
 {
 ?letter <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/knora-base#Resource> .
 GRAPH <http://www.ontotext.com/explicit> {
     ?letter <http://www.knora.org/ontology/knora-base#isDeleted> "false"^^<http://www.w3.org/2001/XMLSchema#boolean> .
 }
 ?letter <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/beol#letter> .
 ?letter <http://www.knora.org/ontology/beol#creationDate> ?date .
 GRAPH <http://www.ontotext.com/explicit> {
     ?date <http://www.knora.org/ontology/knora-base#isDeleted> "false"^^<http://www.w3.org/2001/XMLSchema#boolean> .
 }
 } UNION {
 ?letter <http://www.knora.org/ontology/beol#mentionsPerson> ?person1 .
 ?letter <http://www.knora.org/ontology/beol#mentionsPersonValue> ?person1__LinkValue .
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1__LinkValue <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/knora-base#LinkValue> .
 }
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1__LinkValue <http://www.knora.org/ontology/knora-base#isDeleted> "false"^^<http://www.w3.org/2001/XMLSchema#boolean> .
 }
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1__LinkValue <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> ?letter .
 }
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1__LinkValue <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> ?person1 .
 }
 ?person1 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/knora-base#Resource> .
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1 <http://www.knora.org/ontology/knora-base#isDeleted> "false"^^<http://www.w3.org/2001/XMLSchema#boolean> .
 }
 ?person1 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/beol#person> .
 }
 }
 
 GROUP BY ?letter
 ORDER BY ASC(?letter)
 OFFSET 0
 LIMIT 25

It returns the following results (SPARQL web console):

In Knora, empty results are filtered out:

Since all the variables (keys) are expected to be present in result (a map), Knora throws an exception (ErrorHandlingMap): Status Code: 500 {"status":4,"error":"org.knora.webapi.InconsistentTriplestoreDataException: No value found for SPARQL query variable 'person1Concat' in query result row"}

Proposed solution:

Statements inside an OPTIONAL or a UNION may be present in the results, but they cannot be expected to be there. In scala, we have to check for the key using get and only access it if it is not None.