Oracle: использование подзапроса в триггере

Я думаю, вам нужно что-то sqlselect вроде

CREATE OR REPLACE TRIGGER trigger_w_subquery
AFTER UPDATE OR INSERT ON project_archiving
FOR EACH ROW 
WHEN (old.archiving_status <> new.archiving_status
  AND new.archiving_status = 1)
DECLARE
  l_offer projects.offer%TYPE;
BEGIN
  SELECT offer 
    INTO l_offer
    FROM projects 
   WHERE projnum = :new.projnum;

  IF( l_offer IS NULL )
  THEN
    INSERT INTO offer_log (offer, status, date)
      VALUES (null, 9, sysdate);
  END IF;
END;