[Wikidata-bugs] [Maniphest] T267782: Incorrect use of blank node to represent unknown and no value in Wikidata

[Daniel_Mietchen]

Daniel_Mietchen updated the task description.

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To: Daniel_Mietchen
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[Wikidata-bugs] [Maniphest] T267782: Incorrect use of blank node to represent unknown and no value in Wikidata

[Lucas_Werkmeister_WMDE]

Lucas_Werkmeister_WMDE added a comment.


  In T267782#6645978 , 
@ericP wrote:
  
  > I'll weigh in on 341 after we reach consensus here on whether our (Andra, 
Labra, DanBri, myself, ...) interpretation  is correct that, at least for 
http://www.wikidata.org/entity/Q313093.ttl , a blank node represents the 
unknown value in 
https://www.wikidata.org/wiki/Q313093#Q313093$761f04ee-4d7f-d725-522a-85a3077bb47b
 .
  
  Yes, that’s correct at the moment. (It will eventually change due to T244341 
.)

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[Wikidata-bugs] [Maniphest] T267782: Incorrect use of blank node to represent unknown and no value in Wikidata

[ericP]

ericP added a comment.


  I'll weigh in on 341 after we reach consensus here on whether our (Andra, 
Labra, DanBri, myself, ...) interpretation  is correct that, at least for 
http://www.wikidata.org/entity/Q313093.ttl , a blank node represents the 
unknown value in 
https://www.wikidata.org/wiki/Q313093#Q313093$761f04ee-4d7f-d725-522a-85a3077bb47b
 . (Perhaps Andra can speak to "no value" cases.)

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To: ericP
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[Wikidata-bugs] [Maniphest] T267782: Incorrect use of blank node to represent unknown and no value in Wikidata

[Lucas_Werkmeister_WMDE]

Lucas_Werkmeister_WMDE added a comment.


  But the ultimate motivation for that task is to get rid of //all// blank 
nodes, wherever they appear, because the new query service updater can’t deal 
with them. If you say that we have to use blank nodes for something, then we 
have a big problem – please leave a comment on that task with more details.
  
  (Side note: we don’t use blank nodes for “no value”, and I don’t think we 
ever did. It would be very inconvenient if `FILTER EXISTS { ?item wdt:P40 
?hadAnyChild. }` matched “child: no value”.)

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To: Lucas_Werkmeister_WMDE
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[Wikidata-bugs] [Maniphest] T267782: Incorrect use of blank node to represent unknown and no value in Wikidata

[ericP]

ericP added a comment.


  Oops, sorry, I failed to notice that.
  
  I propose splitting the issue because the considerations are different and I 
expect them to have different outcomes.
  bnodes for OWL constructs is mandated by the OWL specification.
  bnodes for missing and unknown values is a subjective coding choice .

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To: ericP
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[Wikidata-bugs] [Maniphest] T267782: Incorrect use of blank node to represent unknown and no value in Wikidata

[Lucas_Werkmeister_WMDE]

Lucas_Werkmeister_WMDE added a comment.


  T244341  also includes the use of 
blank nodes for unknown values.

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[Wikidata-bugs] [Maniphest] T267782: Incorrect use of blank node to represent unknown and no value in Wikidata

[ericP]

ericP added a comment.


  I don't believe it's the same. T244341: Stop using blank nodes for encoding 
SomeValue and OWL constraints in WDQS 
 is about the use of blank nodes in 
the representation of some OWL expressions. The OWL spec is very clear about 
saying that they have to be blank nodes so the decision was made by the OWL 
Working Group 15 years ago.
  
  This issue is about the use of blank nodes to represent unknown values. The 
example above can be see in 
https://www.wikidata.org/wiki/Q313093#Q313093$761f04ee-4d7f-d725-522a-85a3077bb47b
 which says that Herman Boerhaave's second doctoral advisor is an unknown value 
but a reader of the Turtle page has no way of knowing that. Creating an entity 
for Wikidata-unknown-value would unify coding and make life better for queriers.

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To: ericP
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[Wikidata-bugs] [Maniphest] T267782: Incorrect use of blank node to represent unknown and no value in Wikidata

[Lucas_Werkmeister_WMDE]

Lucas_Werkmeister_WMDE added a comment.


  I’m not sure what this task is about… it sounds like it might be covered by 
T244341: Stop using blank nodes for encoding SomeValue and OWL constraints in 
WDQS ?

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[Wikidata-bugs] [Maniphest] T267782: Incorrect use of blank node to represent unknown and no value in Wikidata

[Andrawaag]

Andrawaag created this task.
Andrawaag added a project: Wikidata.
Restricted Application added a subscriber: Aklapper.

TASK DESCRIPTION
  During the [[ 
https://github.com/elixir-europe/BioHackathon-projects-2020/tree/master/projects/35
 | 
   where we are working on subsetting wikidata, we ran into the issue of blank 
notes being used in the RDF of Wikidata to express unknown and no values. 
Unfortunately, this isn't consistent because blank notes are also used to 
express other things such as owl:complementOf (e.g. Q42).
  
  These blank nodes are also problematic for anything that traverses wikidata 
node-by-node such as faceted browsers or ShEx validators.
  
  It is not explicitly incorrect to have blank nodes in RDF data, but it is:
  
  1. inconsistent with the approach that Wikidata has taken (which is to avoid 
blank nodes)
  2. ambiguous because in RDF, blank nodes do not imply unknown values, they 
are simply *unidentified* nodes in the graph.
  
  Steps to Reproduce:
  
  - GET http://www.wikidata.org/entity/Q313093.ttl
  - look for "_:" (currently _:2d22892344b969be376b57170b5e495f)
  - try a SPARQL query for all properties of that node
  
SELECT ?P ?o { _:2d22892344b969be376b57170b5e495f ?p ?o }
  
  - Because of the semantics of SPARQL, this will try to get every triple in 
the database.
  
  It is not explicitly incorrect to have blank nodes in RDF data, but it is
  
  1. inconsistent with the approach that Wikidata has taken (which is to avoid 
blank nodes)
  2. ambiguous because in RDF, blank nodes do not imply unknown values, they 
are simply *unidentified* nodes in the graph.
  
  Remedy:
  Invent a system-wide identifier for unknown values and use that Q identifier 
for all references to unknown value.

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To: Andrawaag
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