The column list is optional. If you omit the OF column list,
updating any column of table activates the trigger.
The OF column clause is not valid for an INSTEAD OF trigger
on a view.
An UPDATE on the triggering table can activate the trigger in two
cases:
The UPDATE statement references any column in the column list.
The UPDATE event definition has no OF column list specification.
Whether it updates one column or more than one column from the column list,
a triggering UPDATE statement activates each Update trigger only once.
The MERGE statement can also activate an Update trigger,
if the specified table of a trigger with no columns list
is the target table of the MERGE statement, or if the Update clause
of the MERGE statement references a column in the column list
of the Update trigger.