Expressing path expressions over nested quoted triples

[rubensworks]

Why?

A topic that was recently discussed in the RDF-star WG, is the ability to query nested quoted triples at an unknown depth.

For example, given the following dataset:

:Alice :says << :Bob :says << :Carol :says << :Violets :haveColor :Blue >> >> >>
:Alice :says << :Bob :says << :Violets :haveColor :Yellow >> >>
:Alice :says << :Violets :haveColor :Red >>

If one wants to know all claimed colors of :Violets, there is currently no way to express this in a SPARQL (1.2) query, without knowing all nested quoted triple depths beforehand.

Since this topic is part of a broader problem of generalizing path expressions in SPARQL, the WG decided not to design an ad-hoc solution that is specific to paths over nested quoted triples (given the limited charter of the WG), but defer to a future group. Hence, I am opening this issue here to make sure we keep track of it.

Previous work

This issue is related to proposals for extending property path expressions:

• Allow use of variables in property paths #65
• Property paths over graph boundaries #74
• Ability to use predicates (in the XPath sense) in property paths. e.g. "?doc :hasKeyword[dcterms:source :database1]/rdf:value ?keyword" #99
• Reintroduce Property paths forms {x,y} syntax dropped in draft of sparql 1.1 https://www.w3.org/TR/2012/WD-sparql11-query-20120724 #101
• Extend arbitrary length property path to return only furthest node #164
• Support for recursion #184

Proposed solution

Some possible solutions to this specific problem of nested quoted triples can be found below.
But ideally, this should be solved in a more generic manner that captures the requirements from the issues linked above.

Extending property path syntax to allow recursing into nested quoted triples:

:Alice :says*' << :Violets :haveColor ?color >>

An alternative approach that is inspired by the GRAPH syntax
could allow the QUOTED keyword to be used to access any quoted triple (at any nested depth):

SELECT ?person ?color {
  {
    QUOTED ?qOuter {
      ?person :says ?qInner
    }
  } UNION { # Union is necessary because :Alice's statement about :Red is not quoted
    ?person :says ?qInner
  }
  QUOTED ?qInner {
    :Violets :haveColor ?color
  }
}

Considerations for backward compatibility

N/A

[sdasguitar]

Adrian asked me to add the link for the deck of slides that I had presented at the 2022 Knowledge Graph Conference, but I am not sure how I can edit this page. So, I am providing the link here in this comment. Please add it to the "Previous Work" section or wherever appropriate. Thanks.
"Neighborhood-Aware Path Traversal", Souripriya Das.