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Re: "Unbound" in SPARQL (was Re: [TF-LIB] COALESCE is an unhelpful choice of name)

From: Andy Seaborne <andy.seaborne@talis.com>
Date: Tue, 17 Nov 2009 11:35:30 +0000
Message-ID: <4B028A82.4050500@talis.com>
To: Steve Harris <steve.harris@garlik.com>
CC: "public-rdf-dawg@w3.org Group" <public-rdf-dawg@w3.org>
>> This is where the fact relational algebra rename is just a rename (no
>> expressions, it changes the name of a column) makes a difference here.
>> We could special case the translation to the SPARQL algebra:
>> SELECT ?x => project((?x), ....)
>> SELECT (?x AS ?y) => project((?y), rename(?x as ?y) ....))
>> SELECT (?x+1 AS ?y) => project((?y), extend(?x+1 as ?y) ....))
> But the example above was MAX(?x), so it would fall under the "extend"
> semantics anyway, I think.

Yes, it would fall under extend - it returns a value - I was bring up a 
simpler and yet nasty case :-)

We do have the opportunity to define MAX itself. MAX/empty-set can be a 
defined to return term sparql:noValue.  Not a null per se but it might 
work out.  sparql:noValue + 0 is an error.

Received on Tuesday, 17 November 2009 11:35:30 UTC

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