Oracle SYS_CONTEXT用法

1、 USERENV(OPTION)
　　返回当前的会话信息.
　　OPTION='ISDBA'若当前是DBA角色,则为TRUE,否则FALSE.
　　OPTION='LANGUAGE'返回数据库的字符集.
　　OPTION='SESSIONID'为当前会话标识符.
　　OPTION='ENTRYID'返回可审计的会话标识符.
　　OPTION='LANG'返回会话语言名称的ISO简记.
　　OPTION='INSTANCE'返回当前的实例.
OPTION='terminal'返回当前计算机名
　　SELECT USERENV('LANGUAGE') FROM DUAL;

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

oracle dual是一个虚拟表，用来构成select的语法规则，oracle保证dual里面永远只有一条记录.

sql语言不是一个万能的程序语句，它能与外界交互的只有select update insert delete ,这四句都要有表，没表怎么办？那就用dual表吧。