Select name from v$database;
 select sys_context('USERENV','DB_NAME') from dual;
 select global_name from global_name; 
 -- Global name need not be the same as your database name, but it usually is.
 select value from v$parameter where name='db_name';
 select value from v$system_parameter where name='db_name';
 select value from v$spparameter  where name='db_name';