Question

I have a public database link that doesn’t have any uid/pass mentioned. I need to use it in a query but after giving uid/pass.

Something like,
select * from uid/pass.tbl_comp@link_name

what can i do?

Answer 1

As you said,there was no UID and Pass given for a Database Link,then you can use that link as Connected SYSTEM user,where Connected user is defined as:

Connected User: A local user accessing a database link in which
no fixed username and password have been specified. If SYSTEM accesses
a public link in a query, then the connected user is SYSTEM, and the
database connects to the SYSTEM schema in the remote database. Note: A
connected user does not have to be the user who created the link, but
is any user who is accessing the link.

And there are other type of Users category who can use the same link by supplying different parameters while querying the database link,for which you can further read on for usage of Select command with DB link at bottom of this link page:
http://download.oracle.com/docs/cd/B28359_01/server.111/b28310/ds_concepts002.htm

Some Description about Database Link,if you read this before,then please ignore it:

Database Link: A database link connection is one-way in the sense
that a client connected to local database A can use a link stored in
database A to access information in remote database B, but users
connected to database B cannot use the same link to access data in
database A. If local users on database B want to access data on
database A, then they must define a link that is stored in the data
dictionary of database B.

A database link connection allows local users to access data on a
remote database. For this connection to occur, each database in the
distributed system must have a unique global database name in the
network domain. The global database name uniquely identifies a
database server in a distributed system.

Example:-