SPARQL: UNION with two identical branches returns only result for one branch

[cygri]

This SPARQL query should return two solutions, both identical, binding variable ?a to string "a":

SELECT * {
    { BIND ("a" AS ?a) }
    UNION
    { BIND ("a" AS ?a) }
}

But it returns only one of the two solutions. The same effect—only the first union returns any results—happens when the unions have more than one variable or more than one solution each. The problem is triggered by the results being identical, not just by the graph patterns being identical. This one also just returns one solution:

SELECT * {
    { BIND ("a" AS ?a) }
    UNION
    { VALUES ?a { "a" } }
    UNION
    { SELECT ("a" AS ?a) {} }
}

The problem goes away as soon as there is any difference between the results of the two union branches.

[abhivandan1999]

I am interested in solving this issue.

[kushagr08]

@nicholascar
I would like to make a pull request to solve this issue. The result returned by select queries involving union should contain results of both the branches, even if they are identical, as per the w3c union cardinality documentation.
https://www.w3.org/TR/sparql11-query/#defn_algUnion

[nicholascar]

@kushagr08 please, go ahead! @abhivandan1999: you might like to assist.

Please both of you please metion @cygri in your PR so he can review (of course up rdflib maintainers will review as well!).

Make sure in the PR description you say "closes #910" so that when that PR is accepted, this GitHub issue is closed.

[nicholascar]

Oh! @kushagr08, you've already done it in PR #1057! I've added the close statement to that PR description.

Will review that PR shortly.