How to Modify View Oracle character set and how to modify character set

Oracle
[Q] How to view the database character set
[A] Database server character Set select * from Nls_database_parameters, which originates from props$, is the character set representing the database.
Client Character Set Environment select * from Nls_instance_parameters, which originates from V$parameter,
Represents the setting of a client's character set, possibly a parameter file, an environment variable, or a registry
The session Character set environment select * from Nls_session_parameters, which originates from the V$nls_parameters, represents the session's own settings, either the environment variable of the session or the alter sessions complete. If the session does not have a special setting, it will be consistent with nls_instance_parameters.
The client's character set requires consistency with the server in order to correctly display non-ASCII characters of the database. If multiple settings exist, ALTER session> environment variable > registry > Parameter File
Character sets require consistency, but language settings can be different, and language settings are recommended in English. If the character set is ZHS16GBK, then Nls_lang can be AMERICAN_AMERICA.ZHS16GBK.

[Q] How to modify the character set
[a]8i The above version can modify the character set through ALTER DATABASE, but it is also limited to subsets to superset, and it is not recommended to modify the props$ table, which can cause serious errors.)
Startup Nomount;
Alter database Mount exclusive;
Alter system enable restricted session;
Alter system set job_queue_process=0;
Alter database open;
Alter database character Set ZHS16GBK;