Oracle CREATE DATABASE LINK

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:

table_name@database_link
Code language: SQL (Structured Query Language) (sql)

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;
Code language: SQL (Structured Query Language) (sql)

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;
Code language: SQL (Structured Query Language) (sql)

And this query uses the synonym instead of the remote table name with the database link:

SELECT * FROM local_table;
Code language: SQL (Structured Query Language) (sql)

Oracle 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';
Code language: SQL (Structured Query Language) (sql)

In this syntax:

• First, specify the name of the database link after the CREATE DATABASE LINK keywords.
• Second, provide user and password of the remote database after the CONNECT TO and IDENTIFIED BY keywords.
• 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 USING clause.

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))
            )';
Code language: SQL (Structured Query Language) (sql)

To create a public database link, just add the PUBLIC keyword:

CREATE PUBLIC DATABASE LINK dblink 
    CONNECT TO remote_user IDENTIFIED BY password
    USING 'remote_database';
Code language: SQL (Structured Query Language) (sql)

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 SALES.

First, add the following entry to tnsnames.ora file in the local Oracle Database server. Typically, the tnsnames.ora is located in the directory /NETWORK/ADMIN/ under ORACLE_HOME:

SALES =
(DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 10.50.100.143)(PORT = 1521))
    (CONNECT_DATA =
    (SERVER = DEDICATED)
    (SERVICE_NAME = SALES_PRD)
    )
)
Code language: SQL (Structured Query Language) (sql)

Next, use the CREATE DATABASE LINK statement to create a new private database link that connects to the SALES database via bob‘s account:

CREATE DATABASE LINK sales 
    CONNECT TO bob IDENTIFIED BY Abcd1234
    USING 'SALES';
Code language: SQL (Structured Query Language) (sql)

Then, issue the SELECT statement to query data from the customers table on the SALES database:

SELECT * FROM customers@sales;
Code language: SQL (Structured Query Language) (sql)

Here is the output:

After that, insert a new row into the customers table:

INSERT INTO customers@sales(customer_id, name, email)
VALUES(2,'XYZ Inc','contact@xyzinc.com');
Code language: SQL (Structured Query Language) (sql)

Finally, query data from the customers table again:

SELECT * FROM customers@sales
Code language: SQL (Structured Query Language) (sql)

The result set is as follows:

Oracle Database Link best practices

Here are some best practices using the database links:

1. 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 SALES.
2. 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.
3. Use a service-specific entry in the tnsnames.ora instead 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.