察看当前Session登录的数据库

如果可以访问V$表，可以通过
select instance_name, version from v$instance;

如果没有相关权限，也可以通过SYS_CONTEXT得到Session登录的数据库。
select SYS_CONTEXT('USERENV' ,'DB_NAME') db_name from dual;

通过sys_context( namespace, parameter, [ length ] )，可以获取Oracle相关的环境信息。如果需要得到当前session的Oracle相关信息，在namespace的参数值为'USERENV'。还可以通过userenv函数来获取当前session的Oracle相关信息，这是一个遗留函数，Oracle推荐使用sys_context来获取。DBMS_SESSION.set_context设置parameter的值。

select SYS_CONTEXT('USERENV' ,'TERMINAL') terminal from dual;
select SYS_CONTEXT('USERENV' ,'LANGUAGE') language from dual;
select SYS_CONTEXT('USERENV' ,'SESSIONID') sessionid from dual;
select SYS_CONTEXT('USERENV' ,'INSTANCE') instance from dual;
select SYS_CONTEXT('USERENV' ,'ENTRYID') entryid from dual;
select SYS_CONTEXT('USERENV' ,'ISDBA') isdba from dual;
select SYS_CONTEXT('USERENV' ,'NLS_TERRITORY') nls_territory from dual;
select SYS_CONTEXT('USERENV' ,'NLS_CURRENCY') nls_currency from dual;
select SYS_CONTEXT('USERENV' ,'NLS_CALENDAR') nls_calendar from dual;
select SYS_CONTEXT('USERENV' ,'NLS_DATE_FORMAT') nls_date_format from dual;
select SYS_CONTEXT('USERENV' ,'NLS_DATE_LANGUAGE') nls_date_language from dual;
select SYS_CONTEXT('USERENV' ,'NLS_SORT') nls_sort from dual;
select SYS_CONTEXT('USERENV' ,'CURRENT_USER') current_user from dual;
select SYS_CONTEXT('USERENV' ,'CURRENT_USERID') current_userid from dual;
select SYS_CONTEXT('USERENV' ,'SESSION_USER') session_user from dual;
select SYS_CONTEXT('USERENV' ,'SESSION_USERID') session_userid from dual;
select SYS_CONTEXT('USERENV' ,'PROXY_USER') proxy_user from dual;
select SYS_CONTEXT('USERENV' ,'PROXY_USERID') proxy_userid from dual;
select SYS_CONTEXT('USERENV' ,'DB_DOMAIN') db_domain from dual;
select SYS_CONTEXT('USERENV' ,'DB_NAME') db_name from dual;
select SYS_CONTEXT('USERENV' ,'HOST') host from dual;
select SYS_CONTEXT('USERENV' ,'OS_USER') os_user from dual;
select SYS_CONTEXT('USERENV' ,'EXTERNAL_NAME') external_name from dual;
select SYS_CONTEXT('USERENV' ,'IP_ADDRESS') ip_address from dual;
select SYS_CONTEXT('USERENV' ,'NETWORK_PROTOCOL') network_protocol from dual;
select SYS_CONTEXT('USERENV' ,'BG_JOB_ID') bg_job_id from dual;
select SYS_CONTEXT('USERENV' ,'FG_JOB_ID') fg_job_id from dual;
select SYS_CONTEXT('USERENV' ,'AUTHENTICATION_TYPE') authentication_type from dual;
select SYS_CONTEXT('USERENV' ,'AUTHENTICATION_DATA') authentication_data from dual;

执行DBMS_SESSION的SET_CONTEXT函数
http://www.dbazine.com/blogs/blog-cf/chrisfoot/blogentry.2006-11-18.7319544765