I am not a SPARQL expert by any means, so apologies if what seems like a
bug turns out to be misuse on my part.
Using rdfproc 1.0.17 for Debian Stretch I run the following (the second
query from https://www.w3.org/TR/rdf-sparql-query/#alternatives ):
```
rdfproc --contexts --new test add "_:a"
"http://purl.org/dc/elements/1.0/title"; "SPARQL Query Language Tutorial"
rdfproc --contexts test add "_:a"
"http://purl.org/dc/elements/1.0/creator"; "Alice"
rdfproc --contexts test add "_:b"
"http://purl.org/dc/elements/1.1/title"; "SPARQL Protocol Tutorial"
rdfproc --contexts test add "_:b"
"http://purl.org/dc/elements/1.1/creator"; "Bob"
rdfproc --contexts test add "_:c"
"http://purl.org/dc/elements/1.0/title"; "SPARQL"
rdfproc --contexts test add "_:c"
"http://purl.org/dc/elements/1.1/title"; "SPARQL (updated)"
rdfproc --contexts test query sparql - "SELECT ?x ?y WHERE { { ?book
<http://purl.org/dc/elements/1.0/title> ?x } UNION { ?book
<http://purl.org/dc/elements/1.1/title> ?y } }"
```
This gives:
```
result: [, x="SPARQL Query Language Tutorial"]
result: [, x="SPARQL"]
result: []
result: []
```
Notice the lack of `y` bindings.
I have tried these examples with a version compiled from the GitHub
sources and got the same results. I also tried this with the C library,
and it seems to be `librdf_query_results_get_binding_value` that is
failing, although I'm not sure.
Also, FWIW, here's the experiment that made me notice this in the first
place:
```
rdfproc --contexts --new test add s:1 o:readBy i:1 c:1
rdfproc --contexts test add s:1 o:writtenBy i:1 c:2
rdfproc --contexts test add s:3 o:writtenBy i:1 c:3
rdfproc --contexts test add s:3 o:readBy i:1 c:4
rdfproc --contexts test add s:3 o:removedBy i:1 c:5
rdfproc --contexts test query sparql - "SELECT * WHERE { FILTER(?subject
IN (<s:1>, <s:2>, <s:3>)) { GRAPH ?read_context { ?subject <o:readBy>
?read_object } } UNION { GRAPH ?write_context { ?subject <o:writtenBy>
?write_object } } }"
```
Executing the above returns:
```
result: [, subject=<s:1>, read_context=<c:1>, read_object=<i:1>,
write_context=<c:1>]
result: [, subject=<s:3>, read_context=<c:4>, read_object=<i:1>,
write_context=<c:1>]
result: [, subject=<s:1>, read_context=<c:5>]
result: [, subject=<s:3>, read_context=<c:5>]
```
Notice `write_context` is wrong in the first two, `read_context` is
wrong in the second two (`<c:5>` is showing up despite not being asked
for), `write_object` is missing from the first two, and `read_object` is
missing from the second two.
I'd like to finish this though with a huge thanks for librdf and co,
they've been a great help.
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