Question

Just that is the question: is possible to do a ROLLBACK in a MySQL trigger?

If answer is yes, then, please, explain how.

Answer 1

I’ve found out that this functionnality exists since MySQL 5.5 and does not work in earlier releases.

The trigger does no rollback or commit.
To initiate any rollback, you have to raise an exception. Thus your insert/update/delete command will abort.
The rollback or commit action has to be raised around your SQL command.

To raise your exception, in your XXX’s trigger (eg.) :

create trigger Trigger_XXX_BeforeInsert before insert on XXX
for each row begin

    if [Test]
    then

      SIGNAL sqlstate '45001' set message_text = "No way ! You cannot do this !";

    end if ;

end ;