Xquery get the children nodes by passing parent value at run time - xpath

Question

I need to pass a value at run time get all the childen elements.
For example this is my XML:
<person>
<details1>
<name>jack</name>
<age>26</age>
</details1>
<details2>
<name>john</name>
<age>48</age>
</details2>
</person>
And my query:
let $y as xs:string := "details1"
let $x := fn:doc(cts:uri-match('*person.xml'))
return $x/$y
So here I am expecting the result as
<details1>
<name>jack</name>
<age>26</age>
</details1>
but it returns the same name as that of $y i.e. "details1"
or if I query this way
let $y as xs:string := "details1"
let $x := fn:doc(cts:uri-match('*person.xml'))
return $x//$y
the result would be "details1" for 12 times
I am new to XQuery please help me in solving the issue

Answer 1

It seems like you attempt to use a string $y as a node step. However, in XPath/XQuery a path step is different from a string, so you can't simple add a string as path step. Instead, you could look for all descendent elements with the requirement that they do have the same name, i.e.:
let $y as xs:string := "details1"
let $x := fn:doc(cts:uri-match('*person.xml'))
return $x/*[name(.) = $y]

Answer 2

As drkk said, you have to look at the name of the element. But using name() is dangerous. It returns the lexical name of the element, which might be ns:elem if the element has a prefix. And the prefix can be different in every document, even for the same namespace. So either you match only by local name:
let $y as xs:string := 'details1'
let $x := fn:doc(cts:uri-match('*person.xml'))
return
$x/*[local-name(.) eq $y]
Or better, match a QName (note the node-name() usage):
let $y as xs:string := xs:QName('details1')
let $x := fn:doc(cts:uri-match('*person.xml'))
return
$x/*[node-name(.) eq $y]
and if the element name is within a namespace (note the difference between xs:QName() and fn:QName()):
let $y as xs:string := fn:QName('namespace-uri', 'details1')
let $x := fn:doc(cts:uri-match('*person.xml'))
return
$x/*[node-name(.) eq $y]

Answer 3

One alternative is to use xdmp:value() to evaluate the XPath dynamically:
let $y as xs:string := "details1"
let $x := fn:doc(cts:uri-match('*person.xml'))
return xdmp:value("$x/" || $y)
If $x is a large data set, that approach can have performance benefits by removing the need to get the element name for every document in the set.
For more detail, see:
http://docs.marklogic.com/xdmp:value?q=xdmp:value&v=8.0&api=true
Hoping that helps,