Oracle 18c

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CREATE TABLE table_name (Table_name, Parent, PK, Parent_PK) AS
SELECT 'HR',         NULL,         'Id',     NULL     FROM DUAL UNION ALL
SELECT 'Department', 'HR',         'Deptid', 'Id'     FROM DUAL UNION ALL
SELECT 'Emp',        'Department', 'Depid',  'Deptid' FROM DUAL;
3 rows affected

CREATE FUNCTION generate_xml(
  i_name IN VARCHAR2,
  i_pk   IN VARCHAR2
) RETURN XMLTYPE
IS
  v_xml XMLTYPE;
BEGIN
  SELECT XMLELEMENT(
           EVALNAME i_name,
           XMLAGG(
             generate_xml(table_name, pk)
           )
         )
  INTO   v_xml
  FROM   table_name
  WHERE  parent_pk = i_pk;
  RETURN v_xml;
END;
/
SELECT XMLSERIALIZE(
         DOCUMENT generate_xml(table_name, pk)
         AS CLOB
         VERSION '1.0'
       ) AS xml
FROM   table_name
WHERE  parent_pk IS NULL;


XML


<?xml version="1.0"?>
<HR>
  <Department>
    <Emp/>
  </Department>
</HR>