- From: Steve Harris <steve.harris@garlik.com>
- Date: Tue, 5 Jun 2012 06:50:12 -0700
- To: Andy Seaborne <andy.seaborne@epimorphics.com>
- Cc: public-rdf-dawg@w3.org
On 5 Jun 2012, at 03:42, Andy Seaborne wrote: > > On 04/06/12 23:45, Steve Harris wrote: >> On 3 Jun 2012, at 16:03, Andy Seaborne wrote: >> >>> Can aggregate expressions contain aggregates? >>> >>> This comes from a recent email [1] and while these may well not actually address the application goal (a subquery was meant - see thread), the SPARQL 1.1 Query spec does permit these unusual queries. >>> >>> Example: >>> >>> PREFIX ex:<http://example.org/meals#> >>> SELECT (AVG(?mealPrice * (1.0 + MAX( ?mealTip / ?mealPrice))) >>> AS ?avgCostWithBestTip) >>> WHERE { >>> ?description ex:mealPrice ?mealPrice . >>> ?description ex:mealTip ?mealTip . >>> } GROUP BY ?description >>> >>> i.e.an aggregate inside an aggregate: AVG(?x * MAX (?y) ) >>> >>> >>> One line of argument is that the expression inside the aggregate is applied to each row, so only row variables should be considered in-scope. The aggregate AVG(max(?x)+1) is violating that as max(?x) is not a per-row expression. > > (Birte) - yes this needs clarifying if we wish to rule it out, and possible even if we don't. > > As the spec stands, I *think* it says its not allowed: > > [[ > Definition Group: > > Group evaluates a list of expressions against a solution sequence > ... > ]] > > and the solution sequence is the grouped patterns, not after aggregation or select expressions. > > [[ > Definition: Aggregation > ]] > talks about applying the aggregate to the solution sequences collected into a map of key to multiset. > > i.e. the aggregate is evaluated over the pattern, not other aggregates and not select expressions > > Steve - opinion? Yes, I think that the definition says that you wont get a result, but it could probably be clearer about what happens, error / unbound - or do you think it's sufficiently clear? >>> What ARQ does is to calculate the aggregates of a group as the group streams past; it does not wait until the end of evaluation of the whole block when all the elements of all the groups are known. >>> >>> >>> Related to this is the interaction with select expressions: >>> >>> SELECT (max(?x) As ?M) (avg(?M+1) AS ?A) >>> >>> because the select expression rules say you can use ?M inside AVG(). >>> >>> If we wish to forbid this, we can do it quite easily by having a parser rule that aggregates can't appear in expression for the aggregate, which is a simple static check. >> >> Oh boy, it's certainly wacky. >> >> That parse rule wouldn't rule out the use of ?M above though anyway, would it? > > Complicated :-) > > As I read the spec, the ?Ms are different. > > (max(?x) As ?M) -- select expression > > avg(?M+1) -- undefined variable in the grouped pattern that is never mentioned or bound. > > Like writing > > avg(?noSuchVariable+1) Right. > Does any one have a use case that suggests it should be legal? I don't see how it can really have any useful results. - Steve -- Steve Harris, CTO Garlik, a part of Experian 1-3 Halford Road, Richmond, TW10 6AW, UK +44 20 8439 8203 http://www.garlik.com/ Registered in England and Wales 653331 VAT # 887 1335 93 Registered office: Landmark House, Experian Way, NG2 Business Park, Nottingham, Nottinghamshire, England NG80 1ZZ
Received on Tuesday, 5 June 2012 13:51:06 UTC