ORA23515 and ORA02449 on Drop Tablespace

xxd@ETMCDB> alter tablespace ETMCDB offline;
Tablespace altered

xxd@ETMCDB> drop tablespace ETMCDB including contents and datafiles;
drop tablespace ETMCDB including contents
ORA-23515: materialized views and/or their indices exist in the tablespace

xxd@ETMCDB> select 'drop materialized view '||owner||'.'||name||' PRESERVE TABLE;'
              from dba_registered_snapshots where name in  

                   (select table_name from dba_tables where tablespace_name ='ETMCDB');
'DROPMATERIALIZEDVIEW'||OWNER||'.'||NAME||'PRESERVETABLE;'
--------------------------------------------------------------------------------
drop materialized view XXD.MV PRESERVE TABLE;

xxd@ETMCDB> drop materialized view XXD.MV PRESERVE TABLE;
Materialized view dropped.

xxd@ETMCDB> drop tablespace ETMCDB including contents and datafiles;
ORA-02449: unique/primary keys in table referenced by foreign keys

xxd@ETMCDB> drop tablespace ETMCDB including contents and datafiles CASCADE CONSTRAINTS;
Tablespace dropped