Originally posted by Mnemonical

In line triggers (triggers you create
using the FOR EACH ROW sentence) you cannot use DML
(Select, Delete, Insert or Update) commands if the table
you`re using DML is the table the trigger runs the action.
It's called row trigger, not line trigger but that doesn't matter...

Additionnaly the trigger cannot modify the table to which a changing column references via primary, foreign or unique constraint.