Need to concat multiple elements into one element

Question

Hello, here is my challenge:
Query:
SELECT  XMLROOT (
        XMLELEMENT ("Inv",
                         XMLAGG (
                                    XMLELEMENT ("Invline",
                                                 XMLELEMENT ( "Category", i.category ),
                                                XMLELEMENT ( "Amount", i.amount ),
                                     ), VERSION '1.0', STANDALONE YES).EXTRACT ('/*')
  FROM (
        SELECT 1 AS inv_no, 'Labor' AS category, 5 AS amount FROM DUAL
            UNION ALL
        SELECT 1, 'Out-of-Pocket', 10 FROM DUAL
            UNION ALL
        SELECT 1, 'Travel', 20 FROM DUAL
            UNION ALL
        SELECT 1, 'Travel', 2.5 FROM DUAL) i;
Returns:
<?xml version="1.0" standalone="yes"?>
<Inv>
  <Invline>
    <Category>Labor</Category>
    <Amount>5</Amount>
  </Invline>
  <Invline>
    <Category>Out-of-Pocket</Category>
    <Amount>10</Amount>
  </Invline>
  <Invline>
    <Category>Travel</Category>
    <Amount>20</Amount>
  </Invline>
  <Invline>
    <Category>Travel</Category>
    <Amount>2.5</Amount>
  </Invline>
</Inv>
Required:
<?xml version="1.0" standalone="yes"?>
<Inv>
  <Inv_No>1
    <Details>Labor 5
  Out-of-Pocket 10
  Travel 22.5
  </Details>
  </Inv_no>
</Inv>
So I would like help combining the details into one field for transmission to the Mexican Tax Authorities( their spec calls for all  invoice lines to be concated together).
Note that there are two travel details that need to be summed as well.
I would prefer to cannot change the query if possible, just using the XML functions.
Thanks!

Answer 1

And your db version is?
From 11.2 and onwards, you can use LISTAGG aggregate function to concatenate character data across rows :
SELECT XMLElement("Inv"
       , XMLAgg(
           XMLElement("Inv_No"
           , v.inv_no
           , XMLElement("Details"
             , listagg(v.category || ' ' || to_char(v.amount), chr(10))
                 within group (order by v.category)
FROM(
    SELECT t.inv_no
         , t.category
         , sum(t.amount) as amount
    FROM (
      SELECT 1 AS inv_no, 'Labor' AS category, 5 AS amount FROM DUAL UNION ALL
      SELECT 1, 'Out-of-Pocket', 10 FROM DUAL           UNION ALL
      SELECT 1, 'Travel', 20 FROM DUAL            UNION ALL
      SELECT 1, 'Travel', 2.5 FROM DUAL
    ) t
    GROUP BY t.inv_no, t.category
) v
GROUP BY v.inv_no ;