Re: [ACTION-18] use case on !ASK in FILTERS to emulate negation

Ivan Mikhailov wrote:
> Hello Axel,
> 
>> aehm... you won. :-)
>> It starts to look to me like that nesting in FILTERs doesn't really have 
>> any advantages. Anybody arguments in favor of the FILTERed version?
> 
> I'd prefer to not rewrite at all a BI query like
> 
> select ?book ?p1 ?dp1 ?end1 where
>   { ?offer1 :itm ?book ;
>      :price ?p1 .
>      OPTIONAL {
>        ?offer1 :discountPrice ?dp1 ;
>          :endDate ?end1 .
>        filter ((?dp1/?p1 < 0.3) ||
>          ASK { 
>            ?offer2 :itm ?book ;
>            :price ?p2 .
>            OPTIONAL {
>              ?offer2 :discountPrice ?dp2 ;
>                :endDate ?end2 .
>              filter (?end2 >= ?end1) }
>            filter (?dp1/?p1 < ?dp2/?p2) } } ) } }

Let me try to phrase this query into "English"

Give me books and their list prices and optionally their disount prices 
and the date the discount ends if either
  - the discount price is less than a third of the list price, or
  - there is a second offer for that book with a discount that ends
    later, but with a worse ratio of price to discount price?

... aehm, I assume there is a not missing in front of the ASK?
(assuming because otherwise, it is not about Negation anyways, which we 
started)

So, do you mean:

Give me books and their list prices and optionally their disount prices 
and the date the discount ends if either
  - the discount price is less than a third of the list price, or
  - there is no offer which ends later with a better price to
    discount price ratio

?

Was that what you mean? Anyway, thanks for the use case, if clarified, 
it would make sense to add that to the negation use cases on the wiki, 
probably.

Axel

> because the semantics of FILTER in OPTIONAL is "interesting enough":
> it's hard to move the expression to some other place without affecting
> cardinality of the result set. Of course it is possible to replace left
> outer join with UNION but it easily doubles the length of the query for
> each OPTIONAL, mere 5 OPTIONALs would explode the query 32 times.
> 
> Best Regards,
> 
> Ivan Mikhailov
> OpenLink Software
> http://virtuoso.openlinksw.com
> 
> 
> 


-- 
Dr. Axel Polleres
Digital Enterprise Research Institute, National University of Ireland, 
Galway
email: axel.polleres@deri.org  url: http://www.polleres.net/