Get the "issn" attribute from every document in a sequence of arbitrary length:
for $source in $sequence-of-URIs return $doc($source)/*/*:@issn
So you can have an arbitrary number.
It seems that your imaginations are evolving in other directions than the general data processing area I am trying to clarify here.
Do you have a specific use case?
The corresponding understanding is still evolving, isn't it?
The structured query language supports key words like the following. https://en.wikipedia.org/wiki/SQL_syntax#Queries
* … JOIN … ON … * … NATURAL JOIN …
I got the impression that required join conditions need to be specified as predicates in the where clause of the programming language “XQuery” instead. https://www.w3.org/TR/xquery-31/#id-where
Did you occasionally specify join conditions for the desired combination of more than three (or ten) binding sequences (or “tables”) according to a for clause?
Regards, Markus