Author:	Aristoteles
Title:	Avgvstini Niphi In libris Aristotelis meteorologicis commentaria : Eiusdem generalia commentaria in libro de mistis, qui a ueteribus quartus meteororum liber inscribitur, et a iunioribus meteorologicon dicitur. Quibus adiectus est locupletissimus index eorum, quæ tam in textu, quam in commentariis pertractantur, serie alphabetica ordinatus. Omnia multo quam antea emendatiora, accessituta non pauca
Year:	1547
Language:	la
City:	Venetiis
Number of pages:	[13], CXIIII Bl., Bl. 115 - 146 : graph. Darst.

XQuery:

XQueries are executed against its context (a path relative to the context). The context is the document from which the XQuery is executed. XQueries are namespace aware.

Example 1: return all echo:metadata entries of current document
let $result := //echo:metadata/*
return $result

Example 2: count words of first 3 sentences of current document: let $s := //echo:s
let $first3s := $s[position() < 4]
let $first3Str := string-join($first3s, ' ')
let $wordCounter := count(tokenize($first3Str, '\W+')[. != ''])
return $wordCounter
If you want to query over many documents you could do this for example by specifying the document collection and using an XPath expression relative from this collection

Example 3: (XQuery over all documents): return all authors of all archimedes documents ordered let $coll := collection('/db/mpdl/documents/standard/archimedes')
let $result := $coll/archimedes/info/author
let $orderedResult :=
for $elem in $result
order by $elem
return $elem
return $orderedResult

References: XQuery Language 1.0, see here, Wikipedia article, eXist functions, see here

Xpath
XPath queries are executed against its context (a path relative to the context). The context is the document from which the XPath query is executed.

Example: /archimedes/info/author finds only the author of the document from which the query is executed (and not of all documents). XPath queries are namespace aware.

References: XPath Language 2.0, see: here.