在Oracle中，SYS_CONTEXT和USERENV的用法是什么？它们可以返回哪些常用的值？

SYS_CONTEXT函数是Oracle提供的一个获取环境上下文信息的预定义函数。该函数用来返回一个指定NAMESPACE下的PARAMETER值。该函数可以在SQL和PL/SQL语言中使用。

SELECT SYS_CONTEXT('USERENV', 'ACTION') ACTION,

       SYS_CONTEXT('USERENV', 'AUDITED_CURSORID') AUDITED_CURSORID,

       SYS_CONTEXT('USERENV', 'AUTHENTICATED_IDENTITY') AUTHENTICATED_IDENTITY,

       SYS_CONTEXT('USERENV', 'AUTHENTICATION_TYPE') AUTHENTICATION_TYPE,

       SYS_CONTEXT('USERENV', 'AUTHENTICATION_DATA') AUTHENTICATION_DATA,

       SYS_CONTEXT('USERENV', 'AUTHENTICATION_METHOD') AUTHENTICATION_METHOD,

       SYS_CONTEXT('USERENV', 'BG_JOB_ID') BG_JOB_ID,

       SYS_CONTEXT('USERENV', 'CLIENT_IDENTIFIER') CLIENT_IDENTIFIER,

       SYS_CONTEXT('USERENV', 'CLIENT_INFO') CLIENT_INFO, --USERENV('CLIENT_INFO')

       SYS_CONTEXT('USERENV', 'CURRENT_BIND') CURRENT_BIND,

       SYS_CONTEXT('USERENV', 'CURRENT_EDITION_ID') CURRENT_EDITION_ID,

       SYS_CONTEXT('USERENV', 'CURRENT_EDITION_NAME') CURRENT_EDITION_NAME,

       SYS_CONTEXT('USERENV', 'CURRENT_SCHEMA') CURRENT_SCHEMA,

       SYS_CONTEXT('USERENV', 'CURRENT_SCHEMAID') CURRENT_SCHEMAID,

       SYS_CONTEXT('USERENV', 'CURRENT_SQL') CURRENT_SQL,

       SYS_CONTEXT('USERENV', 'CURRENT_SQLN') CURRENT_SQLN,

       SYS_CONTEXT('USERENV', 'CURRENT_SQL_LENGTH') CURRENT_SQL_LENGTH,

       SYS_CONTEXT('USERENV', 'CURRENT_USER') CURRENT_USER,

       SYS_CONTEXT('USERENV', 'CURRENT_USERID') CURRENT_USERID,

       SYS_CONTEXT('USERENV', 'DATABASE_ROLE') DATABASE_ROLE,

       SYS_CONTEXT('USERENV', 'DB_DOMAIN') DB_DOMAIN,

       SYS_CONTEXT('USERENV', 'DB_NAME') DB_NAME,

       SYS_CONTEXT('USERENV', 'DB_UNIQUE_NAME') DB_UNIQUE_NAME,

       SYS_CONTEXT('USERENV', 'DBLINK_INFO') DBLINK_INFO,

       SYS_CONTEXT('USERENV', 'ENTRYID') ENTRYID, --USERENV('ENTRYID')

       SYS_CONTEXT('USERENV', 'ENTERPRISE_IDENTITY') ENTERPRISE_IDENTITY,

       SYS_CONTEXT('USERENV', 'EXTERNAL_NAME') EXTERNAL_NAME,

       SYS_CONTEXT('USERENV', 'FG_JOB_ID') FG_JOB_ID,

       SYS_CONTEXT('USERENV', 'GLOBAL_CONTEXT_MEMORY') GLOBAL_CONTEXT_MEMORY,

       SYS_CONTEXT('USERENV', 'GLOBAL_UID') GLOBAL_UID,

       SYS_CONTEXT('USERENV', 'HOST') HOST, -- USERENV('TERMINAL')

       SYS_CONTEXT('USERENV', 'IDENTIFICATION_TYPE') IDENTIFICATION_TYPE,

       SYS_CONTEXT('USERENV', 'INSTANCE') INSTANCE, --USERENV('INSTANCE')

       SYS_CONTEXT('USERENV', 'INSTANCE_NAME') INSTANCE_NAME,

       SYS_CONTEXT('USERENV', 'IP_ADDRESS') IP_ADDRESS, --ORA_CLIENT_IP_ADDRESS

       SYS_CONTEXT('USERENV', 'ISDBA') ISDBA,  --USERENV('ISDBA')

       SYS_CONTEXT('USERENV', 'LANG') LANG, --USERENV('LANG')

       SYS_CONTEXT('USERENV', 'LANGUAGE') LANGUAGE, --USERENV('LANGUAGE'),

       SYS_CONTEXT('USERENV', 'MODULE') MODULE,

       SYS_CONTEXT('USERENV', 'NETWORK_PROTOCOL') NETWORK_PROTOCOL,

       SYS_CONTEXT('USERENV', 'NLS_CALENDAR') NLS_CALENDAR,

       SYS_CONTEXT('USERENV', 'NLS_CURRENCY') NLS_CURRENCY,

       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', 'NLS_TERRITORY') NLS_TERRITORY,

       SYS_CONTEXT('USERENV', 'OS_USER') OS_USER,

       SYS_CONTEXT('USERENV', 'POLICY_INVOKER') POLICY_INVOKER,

       SYS_CONTEXT('USERENV', 'PROXY_ENTERPRISE_IDENTITY') PROXY_ENTERPRISE_IDENTITY,

       SYS_CONTEXT('USERENV', 'PROXY_USER') PROXY_USER,

       SYS_CONTEXT('USERENV', 'PROXY_USERID') PROXY_USERID,

       SYS_CONTEXT('USERENV', 'SERVER_HOST') SERVER_HOST,

       SYS_CONTEXT('USERENV', 'SERVICE_NAME') SERVICE_NAME,

       SYS_CONTEXT('USERENV', 'SESSION_EDITION_ID') SESSION_EDITION_ID,

       SYS_CONTEXT('USERENV', 'SESSION_EDITION_NAME') SESSION_EDITION_NAME,

       SYS_CONTEXT('USERENV', 'SESSION_USER') SESSION_USER, --ORA_LOGIN_USER

       SYS_CONTEXT('USERENV', 'SESSION_USERID') SESSION_USERID,

       SYS_CONTEXT('USERENV', 'SESSIONID') SESSIONID, --  USERENV('SESSIONID') , V$SESSION.AUDSID

       SYS_CONTEXT('USERENV', 'SID') SID,

       SYS_CONTEXT('USERENV', 'STATEMENTID') STATEMENTID,

       SYS_CONTEXT('USERENV', 'TERMINAL') TERMINAL   --USERENV('TERMINAL')

  FROM DUAL;

USERENV函数用来返回当前的会话信息，常用的有如下信息：

SELECT USERENV('CLIENT_INFO') CLIENT_INFO,

       USERENV('LANGUAGE') 数据库字符集,

       USERENV('ISDBA') 是否DBA角色,

       USERENV('SESSIONID') 当前会话标识符,

       USERENV('ENTRYID') 可审计的会话标识符,

       USERENV('LANG') 会话语言名称的ISO简记,

       USERENV('INSTANCE') 当前的实例,

       USERENV('TERMINAL') 当前计算机名

  FROM DUAL;