select
   XMLDOCUMENT(
      XMLELEMENT(NAME "DoctorPatientRecord",
         XMLFOREST(
         d.doc_id as "DocID", 
         d.doc_lastname as "DocName"
         )
         XMLCONCAT( 
           (select
              XMLELEMENT( NAME "Patient",
                XMLELEMENT(NAME "Firstname", p.pat_firstname),
                XMLELEMENT(NAME "Lastname",  p.pat_lastname)
              )
           from EX.patients p)
         )
      )
   )
FROM EX.patients p, EX.doctors d
where p.doc_id = d.doc_id;