NLS_DATE_FORMAT on database level?

Question

Hi all,
I would like to change the NLS_DATE_FORMAT parameter so that the change will be reflected in the NLS_DATABASE_PARAMETERS view. I always thought it would be alter system, but that only changes it for all newly connecting sessions. Appearantly this can also be set on a database level. What is this database level? What does it mean if the two are different? See details below.
Thanx in advance,
Lennert
SQL> select * from nls_database_parameters where parameter like
SQL> 'NLS_DA%';
PARAMETER                      VALUE
NLS_DATE_FORMAT                DD-MON-RR
NLS_DATE_LANGUAGE              AMERICAN
SQL> select * from nls_session_parameters where parameter like
SQL> 'NLS_DA%';
PARAMETER                      VALUE
NLS_DATE_FORMAT                DD-MM-RR
NLS_DATE_LANGUAGE              DUTCH
SQL> alter system set nls_date_format='DD-MM-YYYY' scope = spfile;
System altered.
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup open
ORACLE instance started.
Database mounted.
Database opened.
SQL>  select * from nls_database_parameters where parameter like
SQL> 'NLS_DA%';
PARAMETER                      VALUE
NLS_DATE_FORMAT                DD-MON-RR      ---> still the same, why?
NLS_DATE_LANGUAGE              AMERICAN     ---> still the same, why?
SQL> select * from nls_session_parameters where parameter like
SQL> 'NLS_DA%';
PARAMETER                      VALUE
NLS_DATE_FORMAT                DD-MM-YYYY   ---> changed
NLS_DATE_LANGUAGE              DUTCH           ---> changed
SQL>

Answer 1

SQL> alter system set nls_date_format='DD-MM-YYYY' scope = spfile;
SQL> select * from nls_database_parameters where parameter like
SQL> 'NLS_DA%';
NLS_DATE_FORMAT DD-MON-RR ---> still the same, why?It is because "alter system" has changed instance parameter and not database parameter.
When you query nls_instance_parameters then you will see that nls_date_format parameter has changed.
Note there are three views:
1. nls_session_parameters - settings at session level, change can be made using "alter session".
2. nls_instance_parameters - settings at instance level, taken from parameter file (pfile/spfile) change can be made using "alter system"
3. nls_database_parameters - settings at database level, set during DB creation, you can't change them.
What is this database level? What does it mean if the two are different? See details below.http://download-uk.oracle.com/docs/cd/B19306_01/server.102/b14220/intro.htm#sthref11
http://download-uk.oracle.com/docs/cd/B19306_01/server.102/b14225/ch3globenv.htm#sthref217