When you start sqlplus withoud SID, Oracle says that you connect to the default database you created during installation. What if I installed multiple databases and I forgot which one is default? Is there a way to find out which is the current default? I wasn't able to find an answer.

But there are a couple of ways to change the default by setting a couple of environment variables. The bad news is: the variables are different depending on which platform you are on.

• Unix
  If you are on Unix and the database is local, set these two variables:
  

  export ORACLE_HOME=/app/oracle/product/10.2.0/db_1
  export ORACLE_SID=orcl
  

  
  
  If the database is remote, set TWO_TASK:
  

  export TWO_TASK=orcl
  

  
  
• Windows
  If you are on Windows, set LOCAL:
  

  set LOCAL=orcl 
  

  
  

Of course, you can always provide SID in the connection string to remove any ambiguity: