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SPARQL When do we use the "a"

I'm new on SPARQL. I read the tutorial on Jena (https://jena.apache.org/tutorials/sparql.html) and I understand most of the example. But I don't really understand when we have to use the a

For example with this triples : 

@prefix vCard:   <http://www.w3.org/2001/vcard-rdf/3.0#> .
@prefix rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .

<http://somewhere/MattJones/>  vCard:FN   "Matt Jones" .
<http://somewhere/MattJones/>  vCard:N    _:b0 .
_:b0  vCard:Family "Jones" .
_:b0  vCard:Given  "Matthew" .


<http://somewhere/RebeccaSmith/> vCard:FN    "Becky Smith" .
<http://somewhere/RebeccaSmith/> vCard:N     _:b1 .
_:b1 vCard:Family "Smith" .
_:b1 vCard:Given  "Rebecca" .

<http://somewhere/JohnSmith/>    vCard:FN    "John Smith" .
<http://somewhere/JohnSmith/>    vCard:N     _:b2 .
_:b2 vCard:Family "Smith" .
_:b2 vCard:Given  "John"  .

<http://somewhere/SarahJones/>   vCard:FN    "Sarah Jones" .
<http://somewhere/SarahJones/>   vCard:N     _:b3 .
_:b3 vCard:Family  "Jones" .
_:b3 vCard:Given   "Sarah" .

We can write something like this : 

PREFIX vcard:      <http://www.w3.org/2001/vcard-rdf/3.0#>

SELECT ?y ?givenName
WHERE
 { ?y vcard:Family "Smith" .
   ?y vcard:Given  ?givenName .
 }

But I the slideshow of my course I found something like this : 

SELECT ?label_du_président ?âge_du_président ? label_du_pays
WHERE {
    ?une_élection a ex:electionPrésidentielle .
    ?une_élection ex:gagnéePar ?un_president .
    ?une_élection ex:alieuDans ?un_pays .
    ?un_president rdfs:label ?label_du_president .
    ?un_president ex:âge ?âge_du_president .
    ?un_pays rdfs:label ?label_du_pays
}

Why and when use the a ?

Solution

  • It's a shorthand notation for the rdf:type property. From the SPARQL Query Language specs:

    The keyword "a" can be used as a predicate in a triple pattern and is an alternative for the IRI http://www.w3.org/1999/02/22-rdf-syntax-ns#type. This keyword is case-sensitive.

    Think of it as meaning "is a (kind of)".