|Oracle9i SQL Reference
Release 2 (9.2)
Part Number A96540-01
SQL Statements: CREATE CLUSTER to CREATE JAVA, 6 of 12
LINK statement to create a database link. A database link is a schema object in the local database that enables you to access objects on a remote database. The remote database need not be an Oracle system.
Once you have created a database link, you can use it to refer to tables and views on the remote database. You can refer to a remote table or view in a SQL statement by appending
@dblink to the table or view name. You can query a remote table or view with the
SELECT statement. If you are using Oracle with the distributed option, you can also access remote tables and views using any
To create a private database link, you must have
LINK system privilege. To create a public database link, you must have
LINK system privilege. Also, you must have
SESSION privilege on the remote Oracle database.
Oracle Net must be installed on both the local and remote Oracle databases.
To access non-Oracle systems you must use Oracle Heterogeneous Services.
SHARED to use a single network connection to create a public database link that can be shared between multiple users.
Oracle9i Heterogeneous Connectivity Administrator's Guide for more information about shared database links
PUBLIC to create a public database link available to all users. If you omit this clause, the database link is private and is available only to you.
Specify the complete or partial name of the database link. The value of the
GLOBAL_NAMES initialization parameter determines whether the database link must have the same name as the database to which it connects.
The maximum number of database links that can be open in one session or one instance of a Real Application Clusters configuration depends on the value of the
OPEN_LINKS_PER_INSTANCE initialization parameters.
Restriction on 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 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
TO clause lets you enable a connection 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 for the link to succeed.
If the database link is used directly, that is, not 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 username that owns the stored object, and not the username 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, the current user is
However, if the stored object is an invoker-rights function, procedure, or package, 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 username and password used to connect to the remote database using a fixed user database link. If you omit this clause, the database link uses the username and password of each user who is connected to the database. This is called a connected user database link.
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.
You must specify this clause when using the
Specify the service name of a remote database.
Oracle9i Net Services Administrator's Guide for information on specifying remote databases
The following statement defines a current-user database link using the sample database:
The following statement defines a fixed-user database link named
Once this database link is created, you can query tables in the schema
scott on the remote database in this manner:
You can also use DML statements to modify data on the remote database:
INSERT INTO firstname.lastname@example.org (customer_id, order_id, order_total) VALUES (5001, 1235, 2000); UPDATE email@example.com SET order_total = order_total + 500; DELETE FROM firstname.lastname@example.org WHERE order_id = 2443;
You can also access tables owned by other users on the same database. This statement assumes that the current user has
SELECT privileges on the
The previous statement connects to the user
scott on the remote database and then queries Adam's
You can create a synonym to hide the fact that the
departments table is on a remote database. The following statement causes all future references to
hr_depts to access a remote
departments table owned by
The following statement defines a shared public fixed user database link named
sales.hq.acme.com that refers to user
hr on the database specified by the string service name '