In many cases you might need to compare data between tables across different databases. For example your test instance and production instance.

You can easily create a database link between the 2 instances and be able to access the production data from the test database.

CREATE DATABASE LINK <Link Name>

 CONNECT TO <Schema Name>

 IDENTIFIED BY <PWD>

 USING '(DESCRIPTION =

    (ADDRESS_LIST =

      (ADDRESS = (PROTOCOL = TCP)(HOST = <Host Name>)(PORT = <port#>))

    )

    (CONNECT_DATA =

      (SERVICE_NAME = <Database Name>)

    )

  )';