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> 1. Editorial
> I'm finding the section on aggregates is quite hard to follow.
> More examples, especially for GROUP_CONCAT, might make it easier
> to understand the link between the algebra and its practical consequences
> in SPARQL queries.

There is a balance between providing examples, and making the document too long.

The explanatory section on aggregates has been expanded, and there are now some testcases for aggregates http://www.w3.org/2009/sparql/docs/tests/data-sparql11/aggregates/ - which hopefully illustrate the behaviour.

> Also, I can't find any indication of the interaction between GROUP BY and ORDER
> BY, where the ORDER BY specifies a variable not named in the GROUP BY: is this
> allowed? If so, what effect does it have (e.g. on GROUP_CONCAT)?

Aggregate operations are performed on a multiset, so order is not preserved.

The group discussed this feature bu decided to postpone this feature to a future working group.

Please note: “ORDER BY after a GROUP BY must only use variables exposed by the GROUP. Use of non-grouping variable outside the GROUP BY is not legal and a required syntax error.”

For example:

  SELECT ?book ?title
    ?book dc:title ?title }
  GROUP BY ?book
  ORDER BY ?title
  ==> error.
> 2. A Nice-to-have feature

We understand that you have now seen another solution to this issue, see http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2010Oct/0001.html.

Could you please respond to this message saying whether this comment has been answered to your satisfaction.