What are the different types of database links used in Oracle?

Connected user
A local user accessing a database link in which no fixed username and password have been specified.
Note that connected users does not have to be the user who created the link, but is any user who is accessing the link.

Example:
CREATE PUBLIC DATABASE LINK sales USING 'sales_db';

Fixed user
A user whose username/password is part of the link definition.

CREATE PUBLIC DATABASE LINK sales CONNECT TO scott IDENTIFIED BY tiger USING 'sales_db';

If you are logged in locally as scott and try to use the database link, a connection as scott is made on the remote side, too.
If the user scott doesn't exist in the remote database, an error will be thrown.

In some cases the database link must contain the entire connect string:

CREATE PUBLIC DATABASE LINK mydatabaselink
CONNECT TO scott
IDENTIFIED BY tiger
USING '(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=remoteserver1.domain.com)(PORT=1521))(CONNECT_DATA= (SID=proddb01)))';

Current user
The link uses the userid/password of the current user to log onto the remote database.

CREATE DATABASE LINK sales CONNECT TO CURRENT_USER USING 'sales';

Note: Current user links are only available through the Oracle Advanced Security option.


Database links are either private or public, authenticated or non-authenticated.
Specify PUBLIC to create a public database link available to all users.

If the value of the GLOBAL_NAMES initialization parameter is TRUE, then the database link must have the same name as the database to which it connects.