Given the following XQuery:

let $input := <root>

                <item><key>a</key><value>1</value></item>

                <item><key>b</key><value>2</value></item>

                <item><key>b</key><value>3</value></item>

              </root>

let $map :=  map:merge((

  map:entry(1, "A"),

  map:entry(2, "B"),

  map:entry(3, "C")

))

for $result in $input/*

let $key := $result/key/text()

let $value := $result/value/text()

group by $key

return map:get($map, $key)

When trying to execute that query, I get the error "Item expected, sequence found: (map {1:"A",2:"B",3:"C"}, map {1:"A",2:"B",3:"C"}).

But why does it group the map? It isn't part of that for-loop but rather some kind of global variable. Or do I misunderstand the "group by" clause?

I only expect the $value to be grouped which results in a sequence.

Use

let $input := <root>
                <item><key>a</key><value>1</value></item>
                <item><key>b</key><value>2</value></item>
                <item><key>b</key><value>3</value></item>
              </root>,

$map :=  map:merge((
  map:entry(1, "A"),
  map:entry(2, "B"),
  map:entry(3, "C")
))
return
for $result in $input/*

to ensure the `group by` clause doesn't change the variable bindings for $input and $map. I am not sure the rest of the code makes sense but you might know better what you want to achieve.