Having defined a service name, the next step in establishing database connectivity is to create database links that take the service name and add a remote user ID and password. Once defined by the DBA, these remote databases can participate in queries and updates from within any Oracle application. For example, databases in London and Paris can be defined to the Denver system with the following SQL extension:

CREATE PUBLIC DATABASE LINK london
 CONNECT TO 'user_id' IDENTIFIED BY 'secret_password'
 USING 'london_unix';
CREATE PUBLIC DATABASE LINK paris
 CONNECT TO 'user_id' IDENTIFIED BY 'secret_password'
  USING 'paris_vms';

Note that the USING clause of the database link specifies a TNS service name.
This service name will be looked up in the tnsnames.ora file to get the protocol, IP address, and the database system ID (SID) name.
The next lesson concludes our examination of Oracle's topology solution by looking at how to join tables from remote sites in a SQL query.