En muchas ocasiones, especialmente en procesos ETL o de carga de datos para un data warehouse, por ejemplo, interesa hacer en una sola sentencia o en un solo paso la comprobación de si un registro existe, y si existe actualizarlo, y si no insertarlo. A esta combinación se le ha apodado UPSERT, aunque en SQL existe una sentencia específica para hacerlo, que es MERGE.

The operation of databases from different manufacturers is similar, but there are many differences between them, even at level of SQL syntax.

For example, if you knows Oracle, and one day you have to do a date conversion with MySQL, you will wonder the following:

- How to do a TO_DATE () / TO_CHAR () with MySQL? 

I open this blog entry to discuss different ways of doing things with each database type. 

Here I link the SQL Tips Bristle Software , which explains quite well how to do some things with MySQL, Oracle and SQL Server, and I find especially useful the chapter on differences between Oracle and SQL Server

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;