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Re: NOT EXISTS vs. MINUS

From: Axel Polleres <axel.polleres@deri.org>
Date: Tue, 29 Mar 2011 10:09:02 +0100
Cc: <public-rdf-dawg-comments@w3.org>
Message-Id: <AD068983-EF69-4640-8662-6D4E1DB9AD2C@deri.org>
To: Jeen Broekstra <jeen.broekstra@gmail.com>
Dear Jeen,

thanks for your thorough and helpful feedback.

This addresses your comment on NOT EXISTS vs. MINUS [1] which 
you later refined in [2]. Your other comments on aggregates 
in [2] will be answered in a separate mail.

Essentially, it seems your concern boils down to the following 
paragraph of your mail:

> This leaves us with two scenarios:
> 
>     1. the two patterns share a variable, in this case the MINUS can be 
>        replaced with a NOT EXISTS;
>     2. the two patterns do not share a variable, in this case the MINUS 
>        can be ignored.
> 
> All in all it seems to me that MINUS as currently defined does not add 
> additional expressivity to the language and is only a syntactic variant. 

Let me first emphasize that the reason we have two negation forms 
is because it reflects two ways of thinking about negation. it's not 
expressivity in the technical sense, it's more about how people think 
about it and this has eventually evolved into two proposals in 
the group which have different syntax.

Next, find an example where MINUS and NOT EXISTS are actually 
different, despite variables are shared. The differences in this 
example are based on NOT EXISTS relying on the variable 
substitution that happens during filter function evaluation:

In this example, the ?n variable isn't bound (isn't in scope) 
in the MINUS block, and so the FILTER inside the MINUS does 
something different than FILTER inside the NOT EXISTS block.

### Data

@prefix : <http://example.com/> . 

:a :p 1 ; :q 1,2 .
:b :p 3.0 ; :q 4.0, 5.0 .

### Query 1: NOT EXISTS

PREFIX : <http://example.com/>
SELECT * WHERE {
	?a :p ?n
	FILTER NOT EXISTS {
		?a :q ?m .
		FILTER(?n = ?m)
	}
}

+------------------------+-----+
| a                      | n   |
+------------------------+-----+
| <http://example.com/b> | 3.0 |
+------------------------+-----+

### Query 2: MINUS

PREFIX : <http://example.com/>
SELECT * WHERE {
	?a :p ?n
	MINUS {
		?a :q ?m .
		FILTER(?n = ?m)
	}
}

+------------------------+-----+
| a                      | n   |
+------------------------+-----+
| <http://example.com/a> | 1   |
| <http://example.com/b> | 3.0 |
+------------------------+-----+
 

Note that such differences are not restricted to cases where 
the shared variables in the MINUS patttern *only* occur 
in a FILTER. E.g., in the example above, make it

	MINUS {
		?a :q ?m .
               OPTIONAL {?a :r ?n} 
               FILTER(?n = ?m))
             } 


Hope this helps for clarification. We'd appreciate if you 
could indicate whether this response adequately addresses 
your comment.

best regards, 
Axel (on behalf of the SPARQL WG)


1. http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2011Jan/0010.html 
2. http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2011Feb/0004.html
Received on Tuesday, 29 March 2011 09:09:36 GMT

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