CREATE DATABASELINK

A database link (DBlink) is a definition of how to establish a connection from one Oracle database to another.
The following link types are supported:

• Private database link - belongs to a specific schema of a database. Only the owner of a private database link can use it.
• Public database link - all users in the database can use it.
• Global database link - defined in an OID or Oracle Names Server. Anyone on the network can use it. 
•  

• SQL> CONNECT test/test
  Connected.
  TEST> CREATE DATABASE LINK VINAY@ROHINI CONNECT TO test IDENTIFIED BY test USING 'mika';
  
  Database link created.
  
  TEST> CONNECT michel/michel
  Connected. 
  MICHEL> ALTER USER test GRANT CONNECT THROUGH michel ;
  
  User altered.
  
  MICHEL> -- Connect to TEST through MICHEL account and so with MICHEL's password
  MICHEL> CONNECT michel[test]/michel
  Connected.
  TEST> SHOW USER
  USER is "TEST" 
  TEST> DROP DATABASE LINK mika@loop;
  
  Database link dropped.
  
  TEST> CONNECT michel/michel
  Connected. 
  MICHEL> ALTER USER test REVOKE CONNECT THROUGH michel ;
  
  User altered.