How to use dbms_metadata to generate DDL for constraints

set long 10000
set heading off
set trimspool on
set lines 300
set longchunksize 300
spool all_mycons.sql
exec dbms_metadata.set_Transform_param(dbms_metadata.session_transform,'SQLTERMINATOR',true);

select dbms_metadata.get_ddl('CONSTRAINT','SYS_C006257121','SCOTT')  FROM DUAL;

-- For Foreign key constraints, use the REF_CONSTRAINT constant:
select dbms_metadata.get_ddl('REF_CONSTRAINT','MYFOREIGNKEY_FK','SCOTT')  FROM DUAL;