I am planning to design a system which can monitor a database (Oracle, MySQL, SQL Server, anyhow). Once a change (insertion of new records, etc.) to the database have been made, a piece of information will be shown.
Is there a way to let the DBMS inform me when anything have been changed.
Triggers is what you need to read about
http://plsql-tutorial.com/plsql-triggers.htm
One idea would be, you can have another table where the triggers will monitor the table and updates that table and your system can only poll that particular table.
Also you can make triggers to send you email and you can poll the email server. Depends on how you design your monitoring system
