Triggers in mySQL 5.0

Link to the article.
What they are:
to check and prevent the insertion of incorrect data
to change or cancel the procedure, INSERT, UPDATE, DELETE
to monitor data changes within sessions.
A trigger is a stored procedure actually executed when a specific event related to a specific data (albeit with some nuances).
Syntax:

And there are specific namespaces OLD and NEW, depending on the context (INSERT, UPDATE, DELETE)

Example of use in order to prevent the insertion of incorrect data