Add /mappings?fromScheme&toScheme lookup

[nichtich]

To get a list of all mappings from Wikidata to a given concept scheme. Result would probably be limited to a maximum number, e.g. 10.000. fromScheme would be optional, set to http://bartoc.org/en/node/1940 (Wikidata) by default.

Sample query: https://w.wiki/7FY

SELECT ?item ?itemLabel ?value ?mappingType WHERE {
  ?item p:P5748 ?statement .
  ?statement ps:P5748 ?value .
  OPTIONAL { ?statement pq:P4390 ?mappingType }
  SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE],en". }
} LIMIT 10000

pagination (LIMIT and OFFSET) requires a sorting, e.g. ORDER BY ?value, this may be too slow for very large concordances such as GND.

[stefandesu]

It would be very good to have pagination available as well, we should at least try to see whether the performance is manageable. With this, mappings could be shown in the Mapping Search as well (even though not all filters will be available).

[nichtich]

Implemented but missing X-Total-Count header and sorting (ORDER BY). Proposed solution:

• get daily property count from https://www.wikidata.org/wiki/Template:Property_uses?action=raw
• if count is relatively low, do SPARQL query to get current number (optional)

[stefandesu]

We can just do a count query no matter how big the count: https://query.wikidata.org/#SELECT%20%28COUNT%28%2a%29%20AS%20%3Fcount%29%0AWHERE%20%7B%0A%20%20%3Fitem%20p%3AP227%20%3Fstatement%0A%7D