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MINUS vs NOT EXISTS question... and some suggestion for rewording.

From: Axel Polleres <axel.polleres@deri.org>
Date: Tue, 5 Oct 2010 16:10:43 -0400
Message-Id: <F3E69456-A174-4D50-B27F-4D61A3160A0E@deri.org>
To: SPARQL Working Group <public-rdf-dawg@w3.org>
http://www.w3.org/2009/sparql/docs/query-1.1/rq25.xml 


In section 8.2 it says:

"The other style of negation provided in SPARQL is
 MINUS which evaluates both its arguments, 
 then calculates solutions in the left-hand side that are not
 compatible with the solutions on the right-hand side."

Isn't this is misleading with respect to the example in Section 8.3 and should rather say ...

"The other style of negation provided in SPARQL is
 MINUS which evaluates both its arguments, 
 then calculates solutions in the left-hand side that either 
 have disjoint domains or are not compatible with the solutions 
 on the right-hand side."


On top of that, just for my understanding, in Section 17.4 we say:

"The additional restriction on dom(&mu;) and dom(&mu;') is added because otherwise
  if there is a solution mapping in &Omega;2 that has no variables in &Omega;1, then
  Minus(&Omega;1, &Omega;2) would be empty, regardless of the rest of &Omega;2"

what does this means on graph 

 G:
  { :a :p :b; c.  :b :p :c}

 and query 
 
 SELECT * WHERE {
    { ?X ?Y ?Z } MINUS { ?A :p ?C OPTIONAL { ?C :p ?X  } }
 }

Further: 

"The empty solution mapping is compatible 
 with every other solution mapping so P MINUS {} 
 would otherwise be empty for any pattern P."

I find that sentence a bit confusing. Maybe it would be good to clarify that then as follows: 

 "The empty solution mapping is compatible, but has disjoint domains with every other solution mapping;
  without the restriction to disjoint domains P MINUS {} would be empty for any pattern P."

best,

Axel
Received on Tuesday, 5 October 2010 21:50:54 GMT

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