Dear Andy,
Thank you for your time. Very appreciated.
Some comments follow in lines.
> On 2 Feb 2016, at 09:36, Andy Seaborne <a...@apache.org> wrote:
>
>
>> when removing the triple (?boss ex:isBossOf ?ind .”), I get
>>
>> SELECT DISTINCT ?ind ?boss ?g
>> WHERE
>> { { ?ind rdf:type ?z }
>> UNION
>> { { ?boss ex:isBossOf1 ?ind }
>> UNION
>> { # Empty BGP
>>
>> }
>> }
>> }
>>
>> which is OK.
>> I just need to find out how to remove an ElementGroup which contains only
>> one element which is the EMPTY one.
>> Of course, I need to do the same for the other case, e.g. OPTION, SUBquery,
>> etc.
>
> Do note that evaluating {} (empty syntax group) yields one row of zero
> columns - it contributes to the overall results (it's the join identity).
>
I see. To avoid this I am going to apply a ElementTransformCleanGroupsOfOne as
you suggested.
> Now you have to look at all the elements that have a group in ElementUnion,
> ElementOptional, ElementMinus, …
Yes, I need to cover all the SPARQL language from the “public Element
transform(ElementGroup arg0, List<Element> arg1)” call.
At least this is my understanding so far.
>
> That is what ElementTransformCleanGroupsOfOne does, except it looks for
> "groups of one"
>
> .. UNION { { stuff } }
>
> and isn't to fussy about finding them all (it's an optimization, more a
> tidying of the tree, not a change in the effect of a query which is what
> removing triple patterns is).
>
> And of course changes from the bottom could potentially cause change all the
> way up to the top of the syntax tree.
>
> also: they maybe be original, legal empty groups in the tree.
>
Thanks for the detailed clarifications. Indeed, I will consider them.
Many Thanks,
Best Regards,
Carlo
> Andy
>
>
>
>
>
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