The Action Clause of INSTEAD OF Triggers
When the trigger event for the specified view is encountered,
the SQL statements of the trigger action are executed, instead of
the triggering statement. Triggers defined on a view support the following
syntax in the action clause.
This is not identical to the syntax of the trigger action for a trigger on a table, as described in the section Triggered Action on a Table. Because no WHEN (condition) is supported, the same trigger action is executed whenever the INSTEAD OF trigger event is encountered, and only one action list can be specified, rather than a separate list for each condition.