First we start up RMAN with a connection to the catalog and the target, making a note of the DBID in the banner:
First we start up RMAN with a connection to the catalog and the target, making a note of the DBID in the banner:

C:>rman catalog=rman/rman@dba1 target=sys/password@w2k1

Recovery Manager: Release 9.2.0.1.0 - Production

Copyright (c) 1995, 2002, Oracle Corporation.  All rights reserved.

connected to target database: W2K1 (DBID=1487421514)
connected to recovery catalog database

RMAN>

Next we list and delete any backupsets recorded in the repository:

RMAN> LIST BACKUP SUMMARY;
RMAN> DELETE BACKUP DEVICE TYPE SBT;
RMAN> DELETE BACKUP DEVICE TYPE DISK;

Next we connect to the RMAN catalog owner using SQL*Plus and issue the following statement:

SQL> CONNECT rman/rman@dba1
Connected.
SQL> SELECT db_key, db_id 
  2  FROM   db 
  3  WHERE  db_id = 1487421514;

    DB_KEY      DB_ID
---------- ----------
         1 1487421514

1 row selected.

SQL>

The resulting key and id can then be used to unregister the database:

SQL> EXECUTE dbms_rcvcat.unregisterdatabase(1, 1487421514);

PL/SQL procedure successfully completed.

SQL>