Summary: in this tutorial, you will learn about the Oracle database link and how to create a database link to a remote Oracle Database server.
What is an Oracle database link
A database link is a connection from the Oracle database to another remote database. The remote database can be an Oracle Database or any ODBC compliant database such as SQL Server or MySQL.
Why do you need a database link
A database link allows a user or program to access database objects such as tables and views from another database.
Once you create a database link, you can access the tables or views from the remote database using the following pattern:
For example, you can query data from a table in the remote database as if it was in the local server:
SELECT * FROM remote_table@database_link;
When accessing a remote table or view over the database link, the Oracle database is acting as an Oracle client.
Using a synonym to simplify the syntax for accessing objects via a database link
To simplify the syntax, you can create a synonym for the remote object accessed via the database link and use this object as if it was a local object.
This syntax shows how to create a synonym for a remote table:
CREATE SYNONYM local_table FOR remote_table@database_link;
And this query uses the synonym instead of the remote table name with the database link:
SELECT * FROM local_table;
CREATE DATABASE LINK statement
There are two types of database links: public and private.
Private database links are visible to the owners while public database links are visible to all users in the database. For this reason, public database links may pose some potential security risks.
To create a private database link, you use the
CREATE DATABASE LINK statement as follows:
CREATE DATABASE LINK dblink CONNECT TO remote_user IDENTIFIED BY password USING 'remote_database';
In this syntax:
- First, specify the name of the database link after the
CREATE DATABASE LINKkeywords.
- Second, provide user and password of the remote database after the
- Finally, specify the service name of the remote database. If you specify only the database name, Oracle will append the database domain to the connect string to form a complete service name.
Typically, you add an entry int the
tnsnames.ora file and reference it as the
remote_database in the
The following statement shows how to create the private database link to a user in a remote database with a full connection string.
CREATE DATABASE LINK dblink CONNECT TO remote_user IDENTIFIED BY password USING '(DESCRIPTION= (ADDRESS=(PROTOCOL=TCP)(HOST=oracledb.example.com)(PORT=1521)) (CONNECT_DATA=(SERVICE_NAME=service_name)) )';
To create a public database link, just add the
CREATE PUBLIC DATABASE LINK dblink CONNECT TO remote_user IDENTIFIED BY password USING 'remote_database';
Create a database link example
In this example, we will create a database link to a remote Oracle Database server located in the server
10.50.100.143 with the port
1521 and service name
First, add the following entry to
tnsnames.ora file in the local Oracle Database server. Typically, the
tnsnames.ora is located in the directory
SALES = (DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = 10.50.100.143)(PORT = 1521)) (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = SALES_PRD) ) )
Next, use the
CREATE DATABASE LINK statement to create a new private database link that connects to the
SALES database via
CREATE DATABASE LINK sales CONNECT TO bob IDENTIFIED BY Abcd1234 USING 'SALES';
Then, issue the
SELECT statement to query data from the
customers table on the
SELECT * FROM customers@sales;
Here is the output:
After that, insert a new row into the
INSERT INTO customers@sales(customer_id, name, email) VALUES(2,'XYZ Inc','firstname.lastname@example.org');
Finally, query data from the
customers table again:
SELECT * FROM customers@sales
The result set is as follows:
Oracle Database Link best practices
Here are some best practices using the database links:
- Naming convention: the name of the database links should reflect the nature of data, not the database server. For example, instead of naming a database link
SALES_PRD, you name it as
- Remote database users: you should create a user dedicated for a database link. In addition, you should not give this user to anyone else. If you don’t follow this, the database will not work when someone changes the password of the user or even delete it.
- Use a service-specific entry in the
tnsnames.orainstead of the database-specific alias so that you copy between product, test, and development environments, you don’t have to recreate the database link.
In this tutorial, you have learned about the Oracle database link concept and how to use the Oracle
CREATE DATABASE LINK to create a new database link to a remote Oracle Database server.