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RE: Semantic Web Phase 2 Activity - Protocol - Query Language

From: Leo Sauermann <leo@gnowsis.com>
Date: Tue, 11 Nov 2003 17:04:35 +0100
To: <mbatsis@netsmart.gr>, <www-rdf-interest@w3.org>
Message-ID: <001b01c3a86d$80ea5b90$0501a8c0@ZION>


Hi,

> >>- OUTER JOIN Queries, (optional matches)
> Saying that a common language should include OUTER JOIN is 
> irrelevant IMHO.

perhaps I did not write correctly what I mean,

because from your answer I think you understood that i wanted to have
some SQL standard stuff written in the Rdf-Query standard. Like "to
implement a RDF-Query, you have to use these internal SQL queries" 
thats not what I meant, the OUTER JOIN problem is a RDF problem you have
when you try to use RDQL (a very popular query language, imho).


If you query
(?x foaf:firstname ?y) (?x foaf:image ?z)

I want to retrieve something like:
x: http://gnowsis.com/leo
y: "Leo"
z: http://gnowsis.com/pictures/leo.jpg 

or

x: http://gnowsis.com/leo
y: "Leo"
z: null

but with today's RDQL this is more complicated:
if there is no <foaf:image> triple for leo, you don't get any data at
all.
:-0
AND THAT IS A PROBLEM, if you have ever programmed RDQL, thats the first
thing you curse.
because you can't retrieve a set of properties, you have to formulate
queries for every single property, its just lame.

see
http://www.google.com/search?q=rdql+outer+join&sourceid=opera&num=0&ie=u
tf-8&oe=utf-8

So the good ol "SQL OUTER JOIN" problem shows up in RDF again....

> may be missing and come up with a document that specifies 
> what the darn 
> language should be able to do against a store, where a store 
> is nothing 
> but a graph or set of graphs.

agreed. thats what a rdf query language is.
Querying RDF (in some result format) from a graph or model or whatever
speaks RDF.



greetings
Leo
Received on Tuesday, 11 November 2003 11:03:57 GMT

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