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Re: Another attempt...

From: Richard Newman <rnewman@twinql.com>
Date: Tue, 18 Mar 2008 16:45:02 -0700
Cc: andy.seaborne@hp.com, "Lee Feigenbaum" <lee@thefigtrees.net>, "Arjohn Kampman" <arjohn.kampman@aduna-software.com>, "public-rdf-dawg-comments@w3.org" <public-rdf-dawg-comments@w3.org>
Message-Id: <932366B2-4596-436A-B6F4-D8D57A016E94@twinql.com>
To: "Andrew Newman" <andrewfnewman@gmail.com>

> Richard made two claims which I was trying to contradict:
> * That {} was unique each time in the query evaluation and

I'm sure that there are many kinds of equality that hold between each  
occurrence of the empty graph pattern in a SPARQL query (string= for  
one). I'm sorry that I failed to get my point across; let me try again.

Each occurrence of {} in the syntactic input is important; you can't  
just discard them. They are unique parsed entities.

They produce unique (i.e., different instances of) empty graph  
patterns in the algebraic output. If I were jumping into Common Lisp,  
each of these empty graph patterns might be EQL or EQUAL, but not EQ.

At this point you no longer have curly brackets, but algebra  
operators. Identities exist in the algebra, but

(Union
   (BGP)
   (BGP))

does not simplify to

(BGP).

> * That the query that involved UNION had some specific order based on
> the syntax.

Check the grammar and the algebra definition -- UNION is a binary  
grammar element (rule 25), and Union is a binary algebra component  
(12.4).

The order of the operands in the algebra expression comes from the  
order of the elements in the parse tree, which comes from the order of  
tokens in the syntactic representation of the query.

> I was hoping someone would say: "In the SPARQL algebra the JOIN
> identity isn't equivalent to this thing in other algebras
> because...??... and that's why it you get the answer when it is
> involved in a UNION in SPARQL".  Defining multiset UNION has in no way
> helped me understand the result.

I think the DAWG is trying to arm you with enough definitions for you  
to phrase an objection using the terms defined in the spec.

The DAWG, and other implementors on the list, have plenty of  
experience with SPARQL; you will have much better luck if you can  
explain an objection as "I think these SPARQL algebra operators should  
work this way"... or, better yet, provide a test case or two.


> Right.  So to concentrate on just this part of the SPARQL document, if
> the name "empty group pattern" is JOIN identity, what would the empty
> set, the UNION identity, be called?  The "even emptier group pattern"
> (as it has a cardinality of 0)?  And how would that look as SPARQL
> syntax perhaps "{-1}"?

If I understand your question correctly: the UNION identity would be a  
failing graph pattern (one that produces no results). Any pattern  
UNIONed with a failing pattern is equivalent to the first pattern.

An example is:

{ FILTER(false) }

but there is no special syntax for this in SPARQL, to my knowledge.

-R
Received on Tuesday, 18 March 2008 23:45:47 GMT

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