- From: Gary King <gwking@metabang.com>
- Date: Wed, 16 Dec 2015 08:57:20 -0500
- To: public-sparql-dev@w3.org
Hi all, I’ve a question on the model query writers should have when reasoning about their work. I think the naive view would be that adding grouping would _not_ alter the meaning of the query but this is not the case. A first example comes from OPTIONAL. A query like select * { ?a :ppp1 ?c . { optional { ?a :ppp2 ?d } } } Is technically equivalent to select * { ?a :ppp1 ?c . ?a :ppp2 ?d . } because the first form looks like (join (bgp ?a :ppp1 ?c) (left-join identity (bgp ?a :ppp2 ?d)) and the left-join will discard no solutions at run-time leaving the outer-most join to merge the two patterns and _discard_ any ?a’s that don’t have a :ppp2 triple. This follows pretty clearly from the SPARQL definition but also seems surprising from a query writers perspective. A second example occurs with BIND. select * { ?a :ppp1 ?c . bind( 2 * ?c as ?twiceC ) } is _not_ the same as select * { ?a :ppp1 ?c . { bind( 2 * ?c as ?twiceC ) } } The second form will leave ?twiceC unbound as ?c has no binding inside of the BIND. Note that this also means that you cannot distribute common patterns out a union because select * { { ?x a ?p . ?p ex:bar ?foo . } union { ?x a ?p . bind(?p as ?foo) } } is _not_ the same as select * { ?x a ?p . { ?p ex:bar ?foo . } union { bind(?p as ?foo) } } If my examples are correct, then SPARQL seems more difficult than it should be to reason about than it should be. I’d welcome comments and thoughts. thanks, -- Gary Warren King, metabang.com Cell: (413) 559 8738 Fax: (206) 338-4052 gwkkwg on Skype * garethsan on AIM * gwking on twitter
Received on Wednesday, 16 December 2015 13:57:55 UTC