CHANGING DATABASE CHARACTER SET

This article describes how one can change the character set of a database.
It should be done with extreme caution having noted the following
limitations.

The database character set should only be changed if the characters of the
code page of the originating database correspond to the same characters
of the target database, ie. if the database was created with the
characterset US7ASCII and it is to be updated to WE8ISO8859P1. Since these
have the same encoding scheme for the first 127 bits, changing the
character set from US7ASCII to WE8ISO8859P1 will display all characters up
to 127 bits as the same character before and after. In addition, in this
particular example, if any characters have been entered with the 8th bit
set, then updating the database characterset to 8 bit will allow that 8th
bit to be displayed. You must not change the characterset from one encoding
scheme to another encoding scheme where the code pages do not correspond.
This will completely scramble your database. In addition, if case*designer
diagrams are stored in the database, this method must not be used. Contact
Worldwide Support for further details.

Before attempting to run any of the scripts below, you must take a full
cold backup of your database. In addition, the procedure must be thoroughly
tested before attempting this on a production instance.

Here's a SQL*PLUS script that allows a database's character set to be
changed to a different encoding scheme without having to rebuild the database.
set echo off
set verify off

rem The data dictionary table that records the database
rem character set is sys.props$
rem
rem SQL> describe sys.props$
rem Name Null? Type
rem ------------------------------- -------- ----
rem NAME NOT NULL VARCHAR2(30)
rem VALUE$ VARCHAR2(2000)
rem COMMENT$ VARCHAR2(2000)

rem For example:
rem
rem SQL> column c1 format a30
rem SQL> select name c1, value$ c1 from sys.props$;

rem C1 C1
rem ------------------------------ ------------------------------
rem DICT.BASE 2
rem NLS_LANGUAGE AMERICAN
rem NLS_TERRITORY AMERICA
rem NLS_CURRENCY $
rem NLS_ISO_CURRENCY AMERICA
rem NLS_NUMERIC_CHARACTERS .,
rem NLS_DATE_FORMAT DD-MON-YY
rem NLS_DATE_LANGUAGE AMERICAN
rem NLS_CHARACTERSET WE8DEC
rem NLS_SORT BINARY
rem GLOBAL_DB_NAME NLSV7.WORLD

rem NLS_CHARACTERSET can be changed by updating its value, for example:

rem update sys.props$
rem set value$ = 'WE8ISO8859P1'
rem Where name = 'NLS_CHARACTERSET';

rem The database has to be shutdown and restarted before the change
rem becomes effective.

rem It is very important to specify the character set name correctly.
rem IMPORTANT NOTE
rem =============
rem If NLS_CHARACTERSET is updated to an invalid value, it will not then
rem be possible to restart the database once it has been shutdown.
rem To recover, it will be necessary to re-create the database, since it
rem cannot be restarted to correct the invalid NLS_CHARACTERSET entry.

rem The character set name should be in uppercase.
rem The new value is not effective until the database has been shutdown and
rem restarted.
rem
rem A suggested procedure is as follows, and can be done by running this
rem script from SQL*Plus when logged into the SYSTEM account.
rem
rem USAGE : SQL> start ch_db.sql
rem
rem where is the desired database character set
rem

Prompt First check that the character set name is valid.

set echo on

select convert('a','&1','us7ascii') from dual;

set echo off

prompt If this select statement returns error ORA-01482, then the
prompt specified character set name is not valid for this installation.
prompt Abort the procedure now with Control-c

prompt To continue, press return
accept ans CHAR

Prompt Check the current value of database character set.

column c1 format a30
select name c1, value$ c1 from sys.props$
where name = 'NLS_CHARACTERSET';

prompt To continue, press return

Prompt Update to new character set

update sys.props$
set value$ = upper('&1')
where name = 'NLS_CHARACTERSET';

set echo off

prompt To continue, press return
accept ans CHAR

Prompt Check the new value of database character set

select name c1, value$ c1 from sys.props$
where name = 'NLS_CHARACTERSET';

Prompt If the value is updated as required, press return to continue and
Prompt then manually type COMMIT; to commit the change. Then shutdown and
Prompt restart the database.
Prompt
Prompt If the value is not updated as required, press return to continue and
Prompt than manually type ROLLBACK; to prevent the change.

prompt To continue, press return
accept ans CHAR