SYS_CONTEXT返回和当前名字空间相关的参数的值，可以使用在SQL/PLSQL中，但不可以用在并行查询或者RAC环境中
语法：
SYS_CONTEXT('NAMESPACE','PARAMETER')
详细的可以参见Oracle参考手册
一些用法：
SQL> show user
USER 为"SCOTT"
SQL> select sys_context('USERENV','HOST') FROM DUAL;
SYS_CONTEXT('USERENV','HOST')
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WORKGROUP\LINUX
SQL> SELECT SYS_CONTEXT('USERENV','SESSIONID') FROM DUAL;
SYS_CONTEXT('USERENV','SESSIONID')
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444
SQL> SELECT SYS_CONTEXT('USERENV','SESSION_USER') FROM DUAL;
SYS_CONTEXT('USERENV','SESSION_USER')
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SCOTT
SQL> SELECT SYS_CONTEXT('USERENV','DB_NAME') FROM DUAL;
SYS_CONTEXT('USERENV','DB_NAME')
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demo
SQL> SELECT SYS_CONTEXT('USERENV','INSTANCE') FROM DUAL;
SYS_CONTEXT('USERENV','INSTANCE')
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1
SQL> SELECT SYS_CONTEXT('USERENV','LANGUAGE') FROM DUAL;
SYS_CONTEXT('USERENV','LANGUAGE')
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SIMPLIFIED CHINESE_CHINA.ZHS16GBK
SQL> SELECT SYS_CONTEXT('USERENV','OS_USER') FROM DUAL;
SYS_CONTEXT('USERENV','OS_USER')
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LINUX\Administrator
SQL>
的确可以得到不少有用的信息
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