- From: Mo Zhou <mozhou@umail.iu.edu>
- Date: Wed, 27 May 2009 13:57:45 -0400
- To: public-rdf-dawg-comments@w3.org
Hi,
I recently read the specification. It is very helpful. However I got
confused by the difference between OPTIONAL and UNION key words. In
all queries using OPTIONAL, we can use UNION to write a query that
generates the same results. and vice versa.
for example:
select ?x ?mbox
where{
?x foaf:name ?name .
optional {?x foaf:mbox ?mbox} .
optional {?x foaf:homepage ?hpage}
}
will generate the same result as
select ?x ?mbox
where{
{?x foaf:name ?name } .
UNION
{?x foaf:mbox ?mbox} .
UNION
{?x foaf:homepage ?hpage}
}
So why the specification includes both OPTIONAL and UNION?
--
Thanks,
Mo
Received on Thursday, 28 May 2009 06:44:10 UTC