SPARQL - What does a single colon do in a prefix?

Question

I've often seen a SPARQL query starting with this prefix:

PREFIX : <http://dbpedia.org/resource/>

But what exactly does it mean to use only a colon ":" in a prefix? I usually know it as putting another abbreviation in front of it. Like for example here:

PREFIX foaf: <http://xmlns.com/foaf/0.1/>

Is there a reason to write it this way, and if so, what is the function of the prefix? I would imagine that this would cover any other abbreviations that were not assigned. But unfortunately I've not found anything specific about this on the Internet

a single colon is also just a prefix name, it's just the empty string — UninformedUser, Dec 02 '22 at 12:33
@UninformedUser Ah ok that makes sense. Could you then use ":prefix" instead of "dbr:prefix" in a query, for example? — Krikell96, Dec 02 '22 at 12:58
well, you mean the correct thing, but the prefix **is** `:` as is `dbr` is a prefix, so it would be `:some` yes. And yes, you can use `:` instead of `dbr:` - it's just that in a query with different entities from different vocabularies sometimes it helps to read `dbr`. In the end a matter of taste I'd say — UninformedUser, Dec 02 '22 at 13:21

score 3 · Accepted Answer · answered Dec 02 '22 at 15:28

There’s no special functionality involved. It’s a regular prefix label, which happens to be empty.

SPARQL: Prefixed Names (bold emphasis mine):

The PREFIX keyword associates a prefix label with an IRI. A prefixed name is a prefix label and a local part, separated by a colon ":". A prefixed name is mapped to an IRI by concatenating the IRI associated with the prefix and the local part. The prefix label or the local part may be empty.

So these three snippets are equivalent:

SELECT * WHERE {
  ?person a <http://xmlns.com/foaf/0.1/Person> .
}

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT * WHERE {
  ?person a foaf:Person .
}

PREFIX : <http://xmlns.com/foaf/0.1/>
SELECT * WHERE {
  ?person a :Person .
}

Using an empty prefix label in SPARQL queries might make sense if all, or almost all, IRIs come from the same ontology, because the query might become more readable then.

SPARQL - What does a single colon do in a prefix?

1 Answers1