In SPARQL, I often see usage of # at the end of prefix definitions, like this:
#prefix dt: <http://example.org/datatype#>
What's the purpose? I couldn't find this in the SPARQL documentation.
Your example seems to be in Turtle, as in SPARQL the syntax would be:
PREFIX dt: <http://example.org/datatype#>
But it’s the same idea: Instead of having to use full IRIs in your query, you can use prefixed names:
In your example, the prefix label is dt. It’s mapped to the IRI http://example.org/datatype#.
In your query, it might get used as dt:foobar, where foobar is called the local part.
The mapped IRI from the prefix label and the local part get concatenated to form the "actual" IRI:
http://example.org/datatype# + foobar =
http://example.org/datatype#foobar
(Instead of using dt:foobar, you could also use <http://example.org/datatype#foobar>.)
So the # just happens to be part of the IRI design. It’s a popular way to structure vocabulary IRIs in the Semantic Web. The other popular way is using a /. See HashVsSlash.
Related
I want to write some reusable SPARQL queries to do things like take an IRI, get the name part (typically after the # sign), modify it (e.g., replace underscores with blank spaces) and put it in the rdfs:label property. This would be useful for Protege which doesn't fill in the rdfs:label if you use user defined IRIs. Or take an IRI with a user defined name, do the same and then replace the user defined IRI with a UUID. I looked in the SPARQL spec for functions to manipulate IRIs and either they don't exist or I'm missing something obvious. I'm posting this to make sure it isn't the latter. I know it is easy to do the equivalent with things like SUBSTR but I'm surprised that there aren't predefined operators to do things like getting the name part of an IRI and getting the base and want to double check before I roll my own.
In case anyone else wants to do this, I figured it out. There are some answers on this site but they are all for SQL or other languages than SPARQL. The following is for classes and it should be obvious how to adapt it for other entities. Note: this works in the Snap SPARQL Plugin for Protege (that's why I use CONSTRUCT rather than INSERT), however, there is a bug in their implementation of SUBSTR so that it uses 0 based indexing rather than 1 based as the spec says. So if you use this in Snap SPARQL change the 1 to a 2.
CONSTRUCT {?c rdfs:label ?lblname.}
WHERE {?c rdfs:subClassOf owl:Thing.
BIND(STRAFTER(STR(?c), '#') as ?name)
BIND(REPLACE(?name,"([A-Z])", " $1" ) as ?namewbs)
BIND (IF (STRSTARTS(?namewbs," "),SUBSTR(?namewbs,1),?namewbs) AS ?lblname)
FILTER(?c != owl:Thing || ?c != owl:Nothing)}
I am currently trying to create pointers to datatype values as they cannot be linked directly. However, I would like to be able to evaluate the pointers from within the SPARQL environment, which raised specifically in the case that the desired value is part of an ordered rdf:List some questions for me. My approach is to use property paths within a SPARQL query in which I can use the defined individual, property and index of the ordered list that the pointer has attached to it.
Given the following example data with the shortened syntax for ordered lists by ttl:
ex:myObject ex:somePropery ("1" "2" "3") .
ex:myPointer ex:lookAtIndividual ex:myObject;
ex:lookAtProperty ex:someProperty ;
ex:lookAtIndex "3"^^xsd:integer .
Now I would like to create a SPARQL query that -- based on the pointer -- returns the value at the given index. To my understanding the query could/should look something like this:
SELECT ?value
WHERE {
ex:myPointer ex:lookAtIndividual ?individual ;
ex:lookAtProperty ?prop ;
ex:lookAtIndex ?index .
?individual ?prop/rdf:rest{?index-1}/rdf:first ?value .
}
But if I try to execute this query with TopBraid, it shows an error message that ?index has been found when <INTEGER> was expected. I also tried binding the index in the SPARQL query via BIND(?index-1 AS ?i), again without success. If the pointed value is not stored in a list, the query without property path works fine.
Is it in general possible to use a value that is connected via datatype property within a SPARQL query as path length for property paths?
This syntax: rdf:rest{<number>} is not standard SPARQL. So the short answer is, regrettably: no, you can't use variables as integers in SPARQL property paths, for the simple reason that you can't use integers in SPARQL property paths at all.
In an earlier draft of the SPARQL standard, there was a proposal to use this kind of syntax to allow specifying the min and max length of a property path, e.g. rdf:rest{1, 3} would match any paths using rdf:rest properties between length 1 and 3. But this was never fully standardized and most SPARQL engines don't implement it.
If you happen to use a SPARQL engine that does implement it, you will have to get in touch with the developers directly to ask if they can extend the mechanism to allow use of variables in this position (the error message suggests to me that it's currently just not possible).
As an aside: there's a SPARQL 1.2 community initiative going on. It only just got started but one of the proposals on the table is re-introducing this particular piece of functionality to the standard.
I got a TSV file that I'm converting with tarql.
Column prop has strings like dc:source, skos:broader etc. How can I convert these to the corresponding URLs? Assume I have all needed prefixes defined in the tarql query.
I can do this statically eg uri(concat(str(dc:),"source")) but how to do it dynamically? The problem can be narrowed to this: given a prefix dc: how to expand it to the appropriate URL?
Looked at ARQ functions but didn't find anything appropriate. If there's no other solution, I can use a VALUES table that repeats the prefixes and namespaces, but what an ugly solution...
The tarql:expandPrefixedName(?qname) function (completely coincidentally committed just today) does exactly what you need: It expands a prefixed name to a full IRI, using any prefixes declared in the query.
The tarql namespace is declared implicitly in every Tarql query.
I find that these are two different query languages:
SPARQL-QUERY and SPARQL-UPDATE.
What other types I could see in SPRARQL?
And I am looking for a syntax where I can replace a particular element property with a new value.
But, using insert query, I can only see that the new value is being added as additional value of the property instead of replacing the whole values of the property.
So, is there any other language for this purpose, like sparql-update something?
Also, I can see that delete option is there. But I don't want to specify a particular value, but to delete the whole pattern. Of course, we can specify the pattern I guess. But I just wonder, if there is a specific language for this purpose.
EDIT:
And in the following query, I don't find the purpose of using where clause at all. It always inserts specified value as a new value, but is not replacing it. We need to use the delete clause specifically. Then what's the purpose of where clause here?
PREFIX dc: <http://purl.org/dc/elements/1.1/>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX indexing: <http://fedora.info/definitions/v4/indexing#>
PREFIX custom: <http://namespaces.info/custom#>
DELETE {
}
INSERT {
<> indexing:hasIndexingTransformation "default";
rdf:type indexing:Indexable;
dc:title "title3";
custom:ownerId "owner6";
dc:identifier "test:10";
}
WHERE {
<>
custom:ownerId "owner1";
}
The SPARQL recommendation is separated into separate documents, see SPARQL 1.1 Overview from W3C.
The WHERE clause can be empty, but also look into INSERT DATA, which takes a set of triple specifications (not patterns - no variables) and inserts them. No WHERE clause id needed int that case. Same for deleting triple specifications with DELETE DATA.
I read this blog article, Problems of the RDF model: Blank Nodes, and there's mentioned that using blank nodes can complicate the handling of data.
Can you give me an example why using blank nodes is difficult to perform a SPARQL query?
I do not understand the complexity of blank nodes.
Can you explain me the meaning and semantics of an existential variable?
I do not understand clearly this explanation given in the RDF Semantics Recommendation, 1.5. Blank Nodes as Existential Variables.
Existential Variables
In the (first-order) predicate calculus, there is existential quantification which lets us make assertions about things that exist, without saying (or, possibly, knowing) which specific individuals in the domain we're actually talking about. For instance, a sentence like
hasUserId(JoshuaTaylor,1281433)
entails the sentence
∃x.hasUserId(x,1281433)
Of course, there are lots of scenarios in which the second sentence could be true without the first one being true. In that sense, the second sentence gives us less information than the first. It's also important to note that the variable x in the second sentence doesn't provide any way to find out which element in the domain of discourse actually has the given userId. It also also doesn't make any claim that there's only one such thing that has the given user id. To make that clearer, we might use an example:
∃y.hasAge(y,29)
This is presumably true, since someone or something out there is age 29. Note that we can't talk about y as the individual that is age 29, though, because there could be lots of them. All this sentence tells us is that there is at least one.
Even though we used different variables in the two sentences, there's nothing to say that the individuals with the specified properties might not be the same. This is particularly important in nested quantification, e.g.,
∃x.∃y.likes(x, y)
This sentence could be true because there is one individual in the domain that likes itself. just because x and y have different names in the sentence doesn't mean that they might not refer to the same individual.
Blank Nodes as Existential Variables
There is a defined RDF entailment model defined in RDF Semantics. This has been described more in another Stack Overflow question, RDF Graph Entailment. The idea is that an RDF graph is treated a big existential quantification over the blank nodes mentioned in the graph. E.g., if the triples in the graph are t1, …, tn, and the blank nodes that appear in those triples are b1, …, bm, then the graph is a formula:
∃b1, …, bm.(t1 ∧ … ∧ tn)
Based on the discussion of the existential variables above, note that this means that blank nodes in the data might refer to same element of the domain, or different elements, and that it's not required that exactly one element could take the place of a blank node. This means that a graph with blank nodes, when interpreted in this manner, provides much less information than you might expect.
Blank Nodes in Real Data
Now, the discussion above is useful if people are using blank nodes as existential variables. In many cases, authors think of them more as anonymous, but definite and distinct objects. E.g., if we casually write
#prefix : <https://stackoverflow.com/q/20629437/1281433/> .
:Carol :hasAddress [ :hasNumber 4222 ;
:hasStreet :Clinton_Way ] .
we may well be trying to say that there is a single address out there with the specified properties, but according to the RDF entailment model, that's not what we're doing.
In practice, this isn't so much of a problem, because we're usually not using RDF entailment. What is a problem though is that since the scope of blank variables is local to a graph, we can't run a SPARQL query against an endpoint asking for Carol's address and get back an IRI that we can reuse. If we run a query like this:
prefix : <https://stackoverflow.com/q/20629437/1281433/>
construct {
:Mike :hasAddress ?address
}
where {
:Carol :hasAddress ?address
}
then we get back the following (unhelpful) graph as a result:
#prefix : <https://stackoverflow.com/q/20629437/1281433/> .
:Mike :hasAddress [] .
We won't have a way to get more information about the address because all we have now is a blank node. If we had used IRIs, e.g.,
#prefix : <https://stackoverflow.com/q/20629437/1281433/> .
:Carol :hasAddress :address1267389 .
:address1267389 :hasNumber 4222 ;
:hasStreet :Clinton_Way .
then the query would have produced something more helpful:
#prefix : <https://stackoverflow.com/q/20629437/1281433/> .
:Mike :hasAddress :address1267389 .
Why is this more useful? The first case is like having the data
∃ x.(hasAddress(Carol,x) ∧ hasNumber(x,4222) ∧ hasStreet(x,ClintonWay))
and getting back a result
∃ y.hasAddress(Mike,y)
Sure, it's possible that Mike and Carol have the same address, but from these sentences there's no way to know for sure. It's much more helpful to have data like
hasAddress(Carol,address1267389)
hasNumber(address1267389,4222)
hasStreet(address1267389,ClintonWay))
and getting back a result
hasAddress(Mike,address1267389)
From this, you know that they have the same address, and you can ask things about it.
Conclusion
How much this will affect your data and its consumers depends on what the typical use cases are. For automatically constructed graphs, it may be hard to know in advance just what kind of data you'll need to be able to refer to later, so it's a good idea to generate IRIs for as many of your resources as you can. Since IRIs are free-form, it's usually not too hard to do this. For instance, if you've got some sensible “base” IRI, e.g.,
http://example.org/myData/
then you can easily append suffixes to identify your resources. E.g.,
http://example.org/myData/addresses/addr1
http://example.org/myData/addresses/addr2
http://example.org/myData/addresses/addr3
http://example.org/myData/individuals/ind34
http://example.org/myData/individuals/ind35