SYS_CONTEXT和USERENV(OPTION)函数

SYS_CONTEXT

Syntax



Description of the illustration sys_context.gif

Purpose

SYS_CONTEXT

returns the value of

parameter

associated with the context

namespace

at the current instant（当前实例）. You can use this function in both SQL and PL/SQL statements.

SYS_CONTEXT

must be executed locally.

the context（[b]context，即上下文，在数据库中是类似于表等概念，是一个数据库对象）[/b]

namespace

must
already have been created, and the associated

parameter

and its value must also have beenset using the

DBMS_SESSION

.

set_context

procedure. The

namespace

must be a valid SQL identifier. The

parameter

name can be any string. It is
not case sensitive, but it cannot exceed 30 bytes in length.

The data type of the return value is

VARCHAR2

. The default maximum size of the return value is 256 bytes(默认返回值的数据类型最大所占字节为256字节). You canoverride
this default （maximum size） by specifying the optional

length

parameter, which must be a

NUMBER

or a value that can be implicitly converted to

NUMBER（参数
length
的数据类型必须是一个[code]NUMBER

类型或是能隐式转换为

NUMBER

的类型[/code]）. The valid range of values is 1 to 4000 bytes（参数length值的范围为1到4000）.
If you specify an invalid value, then Oracle Database ignores it and uses the default.

Oracle provides the following built-in（即内建，也就是说已经预先在数据库中创建了的） namespaces:

USERENV

- Describesthe current session（返回当前会话的信息）. The predefined（预定义） parameters of
namespace

USERENV

are listed in
Table 5-11.

SYS_SESSION_ROLES

- Indicates whethera specified role
is currently enabled for the session. This namespace is available starting with Oracle Database 11g Release 2 (11.2.0.4).

关于context这个数据库对象，参见：

Oracle Database Security Guide for information on using the application context
feature in your application development

CREATE CONTEXT for information on creating user-defined context namespaces（如何创建用户自定义的context）

Oracle Database PL/SQL Packages and Types Reference for
information onthe

DBMS_SESSION

.

set_context

procedure（如何使用[b]the

DBMS_SESSION

.

set_context

来创建并设置[b]

context

[/b]里的参数）[/b]

注释：

context类似于schema这个概念一样，就是一个容器，里面存放多个参数。

Examples

The following statement returns the name of the user who logged onto the database:

CONNECT OE/password
SELECT SYS_CONTEXT ('USERENV', 'SESSION_USER')
FROM DUAL;

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

The following example queries the

SESSION_ROLES

data dictionary view to show that

RESOURCE

is the only role currently enabled for the session. It then uses the

SYS_CONTEXT

function to show that the

RESOURCE

role is currently enabled for the session and the

DBA

role is not.

CONNECT OE/password

SELECT role FROM session_roles;

ROLE
--------
RESOURCE

SELECT SYS_CONTEXT('SYS_SESSION_ROLES', 'RESOURCE')
FROM DUAL

SYS_CONTEXT('SYS_SESSION_ROLES','RESOURCE')
--------------------------------------
TRUE

SELECT SYS_CONTEXT('SYS_SESSION_ROLES', 'DBA')
FROM DUAL;

SYS_CONTEXT('SYS_SESSION_ROLES','DBA')
--------------------------------------
FALSE

The following hypothetical example returns the group number that was set as the value for the attribute

group_no

in the PL/SQL package that was associated with the context

hr_apps

when

hr_apps

was created（假设名为

hr_apps

的context已经创建好了，

group_no

是它的一个参数）:

SELECT SYS_CONTEXT ('hr_apps', 'group_no') "User Group"
FROM DUAL;

附加：

Oracle recommends that you use the [code]SYS_CONTEXT

function with the built-in

USERENV

namespace for current functionality（即USERENV(OPTION)函数）.

译文：

SYS_CONTEXT函数可以以

SYS_CONTEXT ('USERENV', 'OPTION')

取代

USERENV(OPTION)函数的功能。

[/code]

USERENV(OPTION)函数介绍：

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

例子：　　　　SELECT USERENV('LANGUAGE') FROM DUAL;

疑问：

参数 ENTRYID 是什么意思？