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

From: Steve Harris <steve.harris@garlik.com>
Date: Tue, 17 Nov 2009 11:44:48 +0000
Cc: "public-rdf-dawg@w3.org Group" <public-rdf-dawg@w3.org>
Message-Id: <2D78A61A-11B5-41B1-B2E1-35225FEFF349@garlik.com>
To: Andy Seaborne <andy.seaborne@talis.com>
On 17 Nov 2009, at 11:35, Andy Seaborne wrote:

>>> 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.

Right, that may have some slightly strange consequences, and will land  
us more obviously in tri-value logic space, but overall it may be less  
confusing to users. The slightly odd corner cases are things like:

A) SELECT ?x (MAX(?y) AS ?max) ... you get ?x and unbound value for ? 
max - probably expected
B) SELECT ?x (MAX(?y) + 1 AS ?max1) ... you get no row.

But probably users will figure this out. It's reasonably logical. I  
think SQL would always give you the row, as per A), because NULL+1 is  
NULL, not an error.

- Steve, hoping sparql:noValue wasn't supposed to be a URI :)

Steve Harris, CTO, Garlik Limited
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Received on Tuesday, 17 November 2009 11:45:18 UTC

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