Database Software

As far as I know, you have to build the XML yourself via methods such as XMLElement, XMLForest, XMLAgg, etc. Oracle did not have any way to automatically transform the results of a query into XML based upon a registered schema.

Jason_(A_Non) as you suggested i tried to generate xml output/
my query :
SELECT XMLELEMENT("Invoice", XMLAttributes('xmlns="urn:oasis:names:specification:ubl:schema:xsd:Invoice-2"' AS "xmlns:xsi", 'xmlns:cac="urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2"' as "xmlns:cac", 'xmlns:cbc="urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2"' as "xmlns:cbc", 'xmlns:ext="urn:oasis:names:specification:ubl:schema:xsd:CommonExtensionComponents-2"' as "xmlns:ext" ),
XMLAGG(
XMLELEMENT("Invoice",
XMLFOREST(
rc.trx_number AS "cbc:ID",
rc.trx_date AS "cbc:IssueDate",
sum(tax_recoverable) AS "tax_recoverable",
sum(rcg.amount + nvl(tax_recoverable, 0)) AS "net_total"
)
)
)
) as resutl
--INTO l_xmltype
FROM
ra_customer_trx_all rc,
ra_customer_trx_lines_all rcl,
hz_cust_site_uses_all hzcs,
ra_cust_trx_line_gl_dist_all rcg,
gl_code_combinations cc,
HZ_CUST_ACCOUNTS RCA ,
ZX_PARTY_TAX_PROFILE ZPTP,
HZ_PARTIES HZP1
WHERE
rc.customer_trx_id = rcl.customer_trx_id
AND hzcs.site_use_id = rc.bill_to_site_use_id
AND cc.code_combination_id = rcg.code_combination_id
AND rcg.customer_trx_id = rc.customer_trx_id
AND rcl.customer_trx_line_id = rcg.customer_trx_line_id
and rc.SOLD_TO_CUSTOMER_ID=RCA.CUST_ACCOUNT_ID
and RCA.party_id=ZPTP.party_id
and ZPTP.party_id=HZP1.party_id
AND rcg.customer_trx_line_id IS NOT NULL
and line_type='LINE'
--AND rc.customer_trx_id = 8023
group by
rc.trx_number,
HZP1.PARTY_NAME,
'VAT # '||ZPTP.REP_REGISTRATION_NUMBER,
rc.trx_date ;

xml output:
<Invoice xmlns:xsi='xmlns="urn:oasis:names:specification:ubl:schema:xsd:Invoice-2"'
xmlns:cac='xmlns:cac="urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2"'
xmlns:cbc='xmlns:cbc="urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2"'
xmlns:ext='xmlns:ext="urn:oasis:names:specification:ubl:schema:xsd:CommonExtensionComponents-2"'>
<Invoice>
<cbc:ID xmlns:cbc='xmlns:cbc="urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2"'>10002</cbc:ID>
<cbc:IssueDate xmlns:cbc='xmlns:cbc="urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2"'>2019-12-31</cbc:IssueDate>
<tax_recoverable>4125</tax_recoverable>
<net_total>86625</net_total>
</Invoice>
</Invoice>
desire output

<Invoice xmlns:xsi='xmlns="urn:oasis:names:specification:ubl:schema:xsd:Invoice-2"'
xmlns:cac='xmlns:cac="urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2"'
xmlns:cbc='xmlns:cbc="urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2"'
xmlns:ext='xmlns:ext="urn:oasis:names:specification:ubl:schema:xsd:CommonExtensionComponents-2"'>
<Invoice>
<cbc:ID>10002</cbc:ID>
<cbc:IssueDate>2019-12-31</cbc:IssueDate>
<cbc:tax_recoverable>4125</cbc:tax_recoverable>
<cbc:net_total>86625</cbc:net_total>
</Invoice>
</Invoice>

Well, this is part of your answer. I fixed your namespaces as they looked flat out incorrect. I also added the namespace prefix to the two missing nodes

WITH your_from_where_clause (trx_number, trx_date, tax_recoverable, amount) AS
 (SELECT 10002, date '2019-12-31', 4125, 82500 from dual)
SELECT XMLSERIALIZE(document
      XMLELEMENT("Invoice", 
                   XMLAttributes('urn:oasis:names:specification:ubl:schema:xsd:Invoice-2' AS "xmlns:xsi", 
                                 'urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2' as "xmlns:cac", 
                                 'urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2' as "xmlns:cbc", 
                                 'urn:oasis:names:specification:ubl:schema:xsd:CommonExtensionComponents-2' as "xmlns:ext" ),
     XMLAGG(
      XMLELEMENT("Invoice",
       XMLFOREST(
        trx_number AS "cbc:ID",
        trx_date AS "cbc:IssueDate",
        sum(tax_recoverable) AS "cbc:tax_recoverable",
        sum(amount + nvl(tax_recoverable, 0)) AS "cbc:net_total"        
       )
      )
     ) 
    )
    )as resutl
FROM your_from_where_clause
group by 
 trx_number,
 trx_date ;

This version does throw the namespace definition onto every node. While technically valid in terms of XML, I do agree it is annoying. I think the solution is to run the XML through an XSLT to strip off the namespace declaration on all non-root nodes, but I can't seem to find some of those prior examples I know exist so I gave up on the hunt for now.
You have have better luck for this question over in the SQL & PL/SQL forum, simply because more eyes over there. https://community.oracle.com/tech/developers/categories/sql_and_pl_sql