In order to access to the same database where are you working outside the database server you must activate the service called listener, it has to be listening.

It can happens that the database is properly raised and can not connect from other servers, which are also set correctly (correct TNSNAMES, etc.).. 

In these cases could be that the listener has a problem, or simply has not been initiated.

To Check the status, start or stop it is very simple. Just open a command line session (console terminal, etc..) with the user that has installed the database, and run the lsnrctl command with the following parameters:

• Check your state:
    > lsnrctl status

• Stop the listener:
    > lsnrctl stop

• Start the listener:
    > lsnrctl start

Keep in mind that when you stop the listener, the connections that are already in the database won't be closed, so a short stop is not very traumatic, only connections trying to enter while the listener is stopped are rejected, should not affect anyone who already has an opened session.

If you have the system user who installed the database you can enter SQL Plus as DBA user, without entering a password as follows:

1. Enters the system with this user. 
2. From the command line, go into SQLPlus typing:

> sqlplus "/as sysdba"

If you need to enter using this way because you forgot the password of a user, you can easily change it: 

SQL> alter user user_name identified by new_password;

It can be more than one Database installed on the server, so you have to validate that the environment variables of the Oracle's user are pointing to your database.

For verifying that you has login into the correct database you can execute this statement: 

SQL> select name from v$database; 

The easiest way to access from an Oracle database objects from another Oracle database is using a DBLINK (being the easiest does not mean that it is always the most desirable, the abuse of DBLINKS can create many problems, both of performance and safety)

To do this it's necessary a user with CREATE DATABASE LINK privilege, and create a DBLINK in the source database (A) by a simple statement such as:

'LNK_from_A_to_B' is the name of the link, 'USER' and 'PASSWORD' are the IDs of the user who will use the link to connect, which will inherit the permissions of all access through the link, and B is the name of the database's instance.

Using the DBLINK we can connect to the objects with the remote database's permissions that user has been provided in the creation statement.

To reference an object from the remote database should indicate the name of the object, concatenated with the character '@' and the name that we had given to the DBLINK.

Example: 

I tried to create a connection in SQL Developer with a remote SQL Server database. The first steps to configure Oracle SQL Developer are described in the introductory article to SQL Developer, and had already done some testing with a SQL Server installed on the same machine as SQL Developer, but when I tried to do it with a remote BD SQL Server I found some additional difficulty that I think it's interesting to comment.

Great video showing through the use of social media an original story of Jesus' birth at Christmas.

Besides being fun, it's a good way to make a quick tour of the main features of Facebook, Twitter, YouTube, Google, Wikipedia, Google Maps, GMail, Foursquare or Amazon ...

How times change:)

Genial este video que muestra a través de la utilización de medios sociales una original historia del nacimiento de Jesús en Navidad.

Aparte de ser divertido puede ir bien para hacer un recorrido rápido por las principales funcionalidades de Facebook, Twitter, YouTube, Google, Wikipedia, Google Maps, GMail, Foursquare o Amazon...