SPARQL group by and order by: not ordered

Question

I follow up on query where the schema.org database is used to find the number of children of a class - as a simpler database than my application. I want to get the names of the children concatenated in alphabetic order. The query:

prefix schema:  <http://schema.org/>
prefix rdfs:    <http://www.w3.org/2000/01/rdf-schema#>

select    ?child  (group_concat (?string) as ?strings) 
where {
  ?child  rdfs:subClassOf schema:Event .
     ?grandchild rdfs:subClassOf ?child .
  bind (strafter(str(?grandchild), "http://schema.org/") as ?string)
}   group by ?child  order by asc(?string)
limit 20 

gives

schema:PublicationEvent   "OnDemandEvent BroadcastEvent"
schema:UserInteraction    "UserPageVisits UserComments UserPlays UserBlocks UserDownloads UserPlusOnes UserLikes UserCheckins UserTweets"

Which is not alphabetically ordered. If I replace the sort order to desc the result is exactly the same. I seem not to understand how group by , order by and possibly bind interact.

Answer 1

An additional select subquery is required to push the order inside the groups:

prefix schema:  <http://schema.org/>
prefix rdfs:    <http://www.w3.org/2000/01/rdf-schema#>

select    ?child  (group_concat (?string) as ?strings) 

where {
    select * 
    {
     ?child  rdfs:subClassOf schema:Event .
     ?grandchild rdfs:subClassOf ?child .
     bind (strafter(str(?grandchild), "http://schema.org/") as ?string)
    } order by asc(?string)
}   group by ?child  
limit 20 

Answer 2

18.5.1.7 GroupConcat :

The order of the strings is not specified.

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From the horse's mouth :

On 2011-04-22, at 19:01, Steve Harris wrote:

On 2011-04-22, at 06:18, Jeen Broekstra wrote:

However, looking at the SPARQL 1.1 query spec, I think this is not a guaranteed result: as far as I can tell the solution modifier ORDER BY should be applied to the solution sequence after grouping and aggregation, so it can not influence the order of the input for the GROUP_CONCAT .

That's correct.