|Oracle® Database SQL Language Reference
11g Release 2 (11.2)
|PDF · Mobi · ePub|
LINK statement to create a database link. A database link is a schema object in one database that enables you to access objects on another database. The other database need not be an Oracle Database system. However, to access non-Oracle systems you must use Oracle Heterogeneous Services.
After you have created a database link, you can use it in SQL statements to refer to tables, views, and PL/SQL objects in the other database by appending
dblink to the table, view, or PL/SQL object name. You can query a table or view in the other database with the
SELECT statement. You can also access remote tables and views using any
Oracle Database Advanced Application Developer's Guide for information about accessing remote tables or views with PL/SQL functions, procedures, packages, and data types
Oracle Database Administrator's Guide for information on distributed database systems
Oracle Database Reference for descriptions of existing database links in the
USER_DB_LINKS data dictionary views and for information on monitoring the performance of existing links through the
V$DBLINK dynamic performance view
ALTER DATABASE LINK for information on altering a database link when the password of a connection or authentication user changes.
DROP DATABASE LINK for information on dropping existing database links
To create a private database link, you must have the
LINK system privilege. To create a public database link, you must have the
LINK system privilege. Also, you must have the
SESSION system privilege on the remote Oracle Database.
Oracle Net must be installed on both the local and remote Oracle Databases.
PUBLIC to create a public database link visible to all users. If you omit this clause, then the database link is private and is available only to you.
The data accessible on the remote database depends on the identity the database link uses when connecting to the remote database:
If you specify
password, then the database link connects with the specified user and password.
If you specify
CURRENT_USER, then the database link connects with the user in effect based on the scope in which the link is used.
If you omit both of those clauses, then the database link connects to the remote database as the locally connected user.
SHARED to create a database link that can be shared by multiple sessions using a single network connection from the source database to the target database. In a shared server configuration, shared database links can keep the number of connections into the remote database from becoming too large. Shared links are typically also public database links. However, a shared private database link can be useful when many clients access the same local schema, and therefore use the same private database link.
In a shared database link, multiple sessions in the source database share the same connection to the target database. Once a session is established on the target database, that session is disassociated from the connection, to make the connection available to another session on the source database. To prevent an unauthorized session from attempting to connect through the database link, when you specify
SHARED you must also specify the
dblink_authentication clause for the users authorized to use the database link.
See Also:Oracle Database Administrator's Guide for more information about shared database links
Specify the complete or partial name of the database link. If you specify only the database name, then Oracle Database implicitly appends the database domain of the local database.
Use only ASCII characters for
dblink. Multibyte characters are not supported. The database link name is case insensitive and is stored in uppercase ASCII characters. If you specify the database name as a quoted identifier, then the quotation marks are silently ignored.
If the value of the
GLOBAL_NAMES initialization parameter is
TRUE, then the database link must have the same name as the database to which it connects. If the value of
FALSE, and if you have changed the global name of the database, then you can specify the global name.
The maximum number of database links that can be open in one session or one instance of an Oracle RAC configuration depends on the value of the
OPEN_LINKS_PER_INSTANCE initialization parameters.
Restriction on Creating Database Links You cannot create a database link in another user's schema, and you cannot qualify
dblink with the name of a schema. Periods are permitted in names of database links, so Oracle Database interprets the entire name, such as
ralph.linktosales, as the name of a database link in your schema rather than as a database link named
linktosales in the schema
"References to Objects in Remote Databases" for guidelines for naming database links
Oracle Database Reference for information on the
OPEN_LINKS_PER_INSTANCE initialization parameters
"RENAME GLOBAL_NAME Clause" (an
DATABASE clause) for information on changing the database global name
TO clause lets you specify the user and credentials, if any, to be used to connect to the remote database.
CURRENT_USER to create a current user database link. The current user must be a global user with a valid account on the remote database.
If the database link is used directly rather than from within a stored object, then the current user is the same as the connected user.
When executing a stored object (such as a procedure, view, or trigger) that initiates a database link,
CURRENT_USER is the name of the user that owns the stored object, and not the name of the user that called the object. For example, if the database link appears inside procedure
scott.p (created by
scott), and user
jane calls procedure
scott.p, then the current user is
However, if the stored object is an invoker-rights function, procedure, or package, then the invoker's authorization ID is used to connect as a remote user. For example, if the privileged database link appears inside procedure
scott.p (an invoker-rights procedure created by
scott), and user Jane calls procedure
jane and the procedure executes with Jane's privileges.
Specify the user name and password used to connect to the remote database using a fixed user database link. If you omit this clause, then the database link uses the user name and password of each user who is connected to the database. This is called a connected user database link.
You can specify this clause only if you are creating a shared database link—that is, you have specified the
SHARED clause. Specify the username and password on the target instance. This clause authenticates the user to the remote server and is required for security. The specified username and password must be a valid username and password on the remote instance. The username and password are used only for authentication. No other operations are performed on behalf of this user.
Specify the service name of a remote database. If you specify only the database name, then Oracle Database implicitly appends the database domain to the connect string to create a complete service name. Therefore, if the database domain of the remote database is different from that of the current database, then you must specify the complete service name.
See Also:Oracle Database Administrator's Guide for information on specifying remote databases
The examples that follow assume two databases, one with the database name
local and the other with the database name
remote. The examples use the Oracle Database domain. Your database domain will be different.
CREATE PUBLIC DATABASE LINK remote USING 'remote';
This database link allows user
hr on the
local database to update a table on the
remote database (assuming
hr has appropriate privileges):
UPDATE employees@remote SET salary=salary*1.1 WHERE last_name = 'Baer';
CREATE DATABASE LINK local CONNECT TO hr IDENTIFIED BY password USING 'local';
After this database link is created,
hr can query tables in the schema
hr on the
local database in this manner:
SELECT * FROM employees@local;
hr can also use DML statements to modify data on the
INSERT INTO employees@local (employee_id, last_name, email, hire_date, job_id) VALUES (999, 'Claus', 'email@example.com', SYSDATE, 'SH_CLERK'); UPDATE jobs@local SET min_salary = 3000 WHERE job_id = 'SH_CLERK'; DELETE FROM employees@local WHERE employee_id = 999;
Using this fixed database link, user hr on the
remote database can also access tables owned by other users on the same database. This statement assumes that user
SELECT privileges on the
oe.customers table. The statement connects to the user
hr on the
local database and then queries the
SELECT * FROM oe.customers@local;
CREATE DATABASE LINK remote.us.example.com CONNECT TO CURRENT_USER USING 'remote';
The user who issues this statement must be a global user registered with the LDAP directory service.
You can create a synonym to hide the fact that a particular table is on the
remote database. The following statement causes all future references to
emp_table to access the
employees table owned by
hr on the
CREATE SYNONYM emp_table FOR firstname.lastname@example.org;