> Just out of interest, if I was using a pre-XQuery 3.0 processor, would there
> have been any other approach than iterating over distinct-values?
...nope. The query might be evaluated faster if you bin all target
elements to a variable...
let $pii := /dataset/item/pii/string()
for $val in distinct-values($pii)[count($pii[. = $val]) > 1]
return <duplicate>{$val}</duplicate>
...but this depends on the underlying query processor.
> Thanks also for the pointer about using 'null'. The Basex query editor
> seemed to approve it when I tried it once, and the habit stuck ;-)
"null" is a valid XQuery expresion. If you have a database opened,
this query will give you all documents in the database with the root
element <null>...</null>.
