在Oracle中有關資料庫和資料庫例項的幾個重要概念，有時候如果理解不是很深或者對其疏忽、混淆了，還真容易搞錯或弄不清其概念，下面就資料庫例項名、資料庫名、資料庫域名、資料庫服務名、全域性資料庫名幾個概念，我們來梳理一下概念，總結歸納一下這些知識，首先，我們來看看官方文件對這幾者的概念介紹：

 

INSTANCE_NAME（資料庫例項名）

Property	Description
Parameter type	String
Syntax	INSTANCE_NAME =  instance_id
Default value	The instance's SID Note: The SID identifies the instance's shared memory on a host, but may not uniquely distinguish this instance from other instances.
Modifiable	No
Range of values	Any alphanumeric characters
Basic	No

 

In a Real Application Clusters environment, multiple instances can be associated with a single database service. Clients can override Oracle's connection load balancing by specifying a particular instance by which to connect to the database. INSTANCE_NAME specifies the unique name of this instance.

In a single-instance database system, the instance name is usually the same as the database name.

 

 

Oracle Instance是指一組後臺程序（在Windows上是一組執行緒）和一塊共享記憶體區域。例項名（instance_name）就是用來標識這個instance的一個名稱而已。

 

 

DB_NAME（資料庫名）

Property	Description
Parameter type	String
Syntax	DB_NAME = database_name
Default value	There is no default value.
Modifiable	No
Basic	Yes
Real Application Clusters	You must set this parameter for every instance. Multiple instances must have the same value, or the same value must be specified in the STARTUP OPEN SQL*Plus command or the ALTER DATABASE MOUNT SQL statement.

 

DB_NAME specifies a database identifier of up to 8 characters. This parameter must be specified and must correspond to the name specified in the CREATE DATABASE statement.

If you have multiple databases, the value of this parameter should match the Oracle instance identifier of each one to avoid confusion with other databases running on the system. The value of DB_NAME should be the same in both the standby and production initialization parameter files.

The database name specified in either the STARTUP command or the ALTER DATABASE ... MOUNT statement for each instance of the cluster database must correspond to the DB_NAME initialization parameter setting.

The following characters are valid in a database name: alphanumeric characters, underscore (_), number sign (#), and dollar sign ($). No other characters are valid. Oracle removes double quotation marks before processing the database name. Therefore you cannot use double quotation marks to embed other characters in the name. The database name is case insensitive.

 

DB_NAME Initialization Parameter

DB_NAME must be set to a text string of no more than eight characters. During database creation, the name provided for DB_NAME is recorded in the datafiles, redo log files, and control file of the database. If during database instance startup the value of the DB_NAME parameter (in the parameter file) and the database name in the control file are not the same, the database does not start.

 

簡單來說，資料庫名是資料庫的名稱標識，它是在建立資料庫的時候確定的，一旦確定，不能更改。該資訊存在於初始化檔案，控制檔案、redo log檔案以及資料檔案等地方。

 

 

 

DB_DOMAIN（資料庫域名）

Property	Description
Parameter type	String
Syntax	DB_DOMAIN = domain_name
Default value	There is no default value.
Modifiable	No
Range of values	Any legal string of name components, separated by periods and up to 128 characters long (including the periods). This value cannot be NULL.
Basic	Yes
Real Application Clusters	You must set this parameter for every instance, and multiple instances must have the same value.

 

In a distributed database system, DB_DOMAIN specifies the logical location of the database within the network structure. You should set this parameter if this database is or ever will be part of a distributed system. The value consists of the extension components of a global database name, consisting of valid identifiers (any alphanumeric ASCII characters), separated by periods. Oracle recommends that you specify DB_DOMAIN as a unique string for all databases in a domain.

This parameter allows one department to create a database without worrying that it might have the same name as a database created by another department. If one sales department's DB_DOMAIN is JAPAN.ACME.COM, then their SALES database (SALES.JAPAN.ACME.COM) is uniquely distinguished from another database with DB_NAME = SALES but with DB_DOMAIN = US.ACME.COM.

If you omit the domains from the name of a database link, Oracle expands the name by qualifying the database with the domain of your local database as it currently exists in the data dictionary, and then stores the link name in the data dictionary. The characters valid in a database domain name are: alphanumeric characters, underscore (_), and number sign (#).

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