-- https://docs.oracle.com/en/database/oracle/oracle-database/23/sqlrf/XMLCDATA.html
SELECT XMLELEMENT("PurchaseOrder",
   XMLAttributes(dummy as "pono"),
   XMLCdata('<!DOCTYPE po_dom_group [
   <!ELEMENT po_dom_group(student_name)*>
   <!ELEMENT po_purch_name (#PCDATA)>
   <!ATTLIST po_name po_no ID #REQUIRED>
   <!ATTLIST po_name trust_1 IDREF #IMPLIED>
   <!ATTLIST po_name trust_2 IDREF #IMPLIED>
   ]>')) "XMLCData" FROM DUAL;