====== Oracle - SQL - Trigger - Create or Replace Trigger ======

<code sql>
CREATE OR REPLACE TRIGGER trigger-name
BEFORE INSERT ON table-name
FOR EACH ROW
WHEN (NEW.field-name IS NULL OR NEW.field-name = '<new>')
BEGIN
SELECT 'PR-' || sequence-name.NEXTVAL INTO :NEW.field-name FROM DUAL;
END;
</code>