(1) ADQL is semantically similar to SQL, it is something for querying
relational databases like star catalogs -- eg "find all rows in the table
where alpha > 3". ADQL extends SQL because it has concepts of coverage and
proximity.
(2) Xquery is for querying hierarchical databases (like XML documents). (see
example* below). Xquery is made for XML (which VOResource is), and there are
existing implementations.
I believe that we are agreed that an SQL-type language is best for
relational databases of stars, and that our agreement from Cambridge covers
this. However, the registry is different from a star catalog in several ways
(a) the structure is more complex, with repeated elements (eg authors), and
(b) the amount of data is much less. In other words, the registry is
*metadata*, not *data*. There is not much experience on querying these --
because we do not actually have any significant VO registries yet!
Querying the registry is not a solved problem (like SQL and star catalogs). There is no benefit in entrenching ourselves in a position. I think we in the IVOA should continue to investigate ways to make client software for VO registries, and hope that by this time next year we will be sufficiently informed to consider a standards process.
Roy
<query>
{ for $b in document("nvo.caltech.edu/registry.xml")/VOResource
where $b/Curation/Creator="Messier" and $b/@date<1800
return <resource year="{$b/@date}"> {$b/Curation/Title} </resource> }
</query>