还是看sql参考文档吧：
Creating Database Links
You create a database link with the statement CREATE DATABASE LINK . The statement lets you specify this information about the database link:
The name of the database link
The database connect string to access the remote database
The username and password to connect to the remote database
Oracle stores this information in the data dictionary.
Database Link Names
When you create a database link, you must specify its name. Database link names are different from names of other types of objects. They can be as long as 128 bytes and can contain periods (.) and the "at" sign (@).
The name that you give to a database link must correspond to the name of the database to which the database link refers and the location of that database in the hierarchy of database names. The following syntax diagram shows the form of the name of a database link:
dblink::=
Description of the illustration dblink.gif
where:
database should specify the name portion of the global name of the remote database to which the database link connects. This global name is stored in the data dictionary of the remote database; you can see this name in the GLOBAL_NAME data dictionary view.
domain should specify the domain portion of the global name of the remote database to which the database link connects. If you omit domain from the name of a database link, then Oracle qualifies the database link name with the domain of your local database as it currently exists in the data dictionary.
connect_descriptor lets you further qualify a database link. Using connect descriptors, you can create multiple database links to the same database. For example, you can use connect descriptors to create multiple database links to different instances of the Real Application Clusters that access the same database.
The combination database.domain is sometimes called the service name.
See Also:
Oracle Net Services Administrator's Guide
Username and Password
Oracle uses the username and password to connect to the remote database. The username and password for a database link are optional.
Database Connect String
The database connect string is the specification used by Oracle Net to access the remote database. For information on writing database connect strings, see the Oracle Net documentation for your specific network protocol. The database string for a database link is optional.
Referring to Database Links
Database links are available only if you are using Oracle distributed functionality. When you issue a SQL statement that contains a database link, you can specify the database link name in one of these forms:
The complete database link name as stored in the data dictionary, including the database, domain, and optional connect_descriptor components.
The partial database link name is the database and optional connect_descriptor components, but not the domain component.
Oracle performs these tasks before connecting to the remote database:
If the database link name specified in the statement is partial, then Oracle expands the name to contain the domain of the local database as found in the global database name stored in the data dictionary. (You can see the current global database name in the GLOBAL_NAME data dictionary view.)
Oracle first searches for a private database link in your own schema with the same name as the database link in the statement. Then, if necessary, it searches for a public database link with the same name.
Oracle always determines the username and password from the first matching database link (either private or public). If the first matching database link has an associated username and password, then Oracle uses it. If it does not have an associated username and password, then Oracle uses your current username and password.
If the first matching database link has an associated database string, then Oracle uses it. Otherwise Oracle searches for the next matching (public) database link. If no matching database link is found, or if no matching link has an associated database string, then Oracle returns an error.
Oracle uses the database string to access the remote database. After accessing the remote database, if the value of the GLOBAL_NAMES parameter is true, then Oracle verifies that the database.domain portion of the database link name matches the complete global name of the remote database. If this condition is true, then Oracle proceeds with the connection, using the username and password chosen in Step 2. If not, Oracle returns an error.
If the connection using the database string, username, and password is successful, then Oracle attempts to access the specified object on the remote database using the rules for resolving object references and referring to objects in other schemas discussed earlier in this section.
You can disable the requirement that the database.domain portion of the database link name must match the complete global name of the remote database by setting to false the initialization parameter GLOBAL_NAMES or the GLOBAL_NAMES parameter of the ALTER SYSTEM or ALTER SESSION statement.
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