SPARQL: How to obtain label in available languages if first option is not available

问题

If a Wikidata resource returned by my query has no available label in the language I filtered for I obtained an empty cell.

SELECT *
WHERE
{
    ?country wdt:P31 wd:Q6256.
    ?country rdfs:label ?country_name
        FILTER(LANG(?country_name) = 'jbo').
}

link

How to request to have a label returned in one of any of the available languages if the first language fails?

回答1:

First, prefer langMatches for checking language tags. This is especially important in your case, since you might want, for instance, a label in English, and langMatches(lang(?label), "en") will find a label with the tag "en", or "en-GB", or "en-US", etc. Those are regional variants for the language, and langMatches can help you find them.

Updated Solution based on comments

@svick noticed in the comments that the original solution ends up with a row for each element in the Cartesian product of the English names with the non-English names. You can avoid that by using a select distinct. But there's really a better way: just use the same variable in two optionals; the first checks for an English label, and the second checks for non-English labels. If the first succeeds, then the second never gets invoked. That is, just do:

select ?country ?label {
   ?country wdt:P31 wd:Q6256 
   optional { 
     ?country rdfs:label ?label
     filter langMatches(lang(?label), "en")
   }
   optional { 
     ?country rdfs:label ?label
   }
}

Other options

If you need to do any aggregation, you may find some help in SPARQL filter language if possible in multiple value context.
If, after the first language, you still have preferences on the remaining languages, you may find the technique used in Sparql multi lang data compression to one row helpful.

Original Solution with COALESCE

After that, though, coalesce will do what you want. It takes a number of arguments, and returns the first one that has a value. So, you can match the preferred language in an optional block, and any language in another, and coalesce the values:

select distinct ?country (coalesce(?enLabel, ?anyLabel) as ?label) {
   ?country wdt:P31 wd:Q6256 
   optional { 
     ?country rdfs:label ?enLabel
     filter langMatches(lang(?enLabel), "en")
   }
   optional { 
     ?country rdfs:label ?anyLabel
   }
}

来源：https://stackoverflow.com/questions/38349975/sparql-how-to-obtain-label-in-available-languages-if-first-option-is-not-availa

标签

sparql

linked-data

wikidata