Missing data fields in Wikidata Query Service results despite items having these data

Question

I am trying to retrieve some municipalities from Wikidata using SPARQL but several items returned have much of their fields empty despite these items having these data. I do not understand what is wrong with the query below (link to WQS). For example, the municipality Almelo has its coordinates (P625), and parent place (P131) erroneously missing in the results:

SELECT ?mun ?munLabel ?coords ?parentPlace ?area WHERE {
  ?mun p:P31 ?instanceOf # Get statement because we need this later
       .
  ?instanceOf ps:P31/wdt:279* wd:Q2039348.
  
  OPTIONAL {
    ?mun wdt:P625 ?coords;
      wdt:P131 ?parentPlace; 
      wdt:P2046 ?area
      .
  }
  
  MINUS { ?instanceOf pq:P582 ?endTime. } # Don't show municipalities that have an end time
  
  service wikibase:label { bd:serviceParam wikibase:language "en". }
} ORDER BY ?munLabel

Answer 1

This is because you are using one OPTIONAL statement instead of 3 separately. In this case, Almelo doesn't have an 'area', wdt:P2046, so the whole OPTIONAL statement evaluates as false, and so it binds no variables.

The following query works: Notice that we have 3 distinct optional statements, so that they may fail to bind variables independently of each other.

SELECT ?mun ?munLabel ?coords ?parentPlace ?area WHERE {
  ?mun p:P31 ?instanceOf # Get statement because we need this later
       .
  ?instanceOf ps:P31/wdt:279* wd:Q2039348.
  
  OPTIONAL {?mun wdt:P625 ?coords }
  OPTIONAL {?mun wdt:P131 ?parentPlace }
  OPTIONAL {?mun wdt:P2046 ?area }

  
  MINUS { ?instanceOf pq:P582 ?endTime. } # Don't show municipalities that have an end time
  
  service wikibase:label { bd:serviceParam wikibase:language "en". }
} ORDER BY ?munLabel

Answer 2

You have to declare OPTIONAL each statement independently:

  OPTIONAL { ?mun wdt:P625 ?coords . }
  OPTIONAL { ?mun wdt:P131 ?parentPlace . }
  OPTIONAL { ?mun wdt:P2046 ?area . }

Otherwise, if one of them is missing, then the whole OPTIONAL block is ignored.

See also Multiple Optional Graph Patterns.