Subject Referential integrity with triggers

( I hope my post won't be in HTML; posting directly from yahoo web
site )

According to a previous post (from Helen Borrie), it is a better
choice to implement RI with triggers (rather than FK) on "lookup
links", corresponding to low selectivity columns.

Question: how can a (before insert) trigger prevent from inserting an
ORDER if the corresponding CUSTOMER_ID does not exist?
Or: how can a (before delete) trigger prevent from deleting a
CUSTOMER as long as ORDERs still exist?

Does someone have examples?

Fabrice Aeschbacher