My trigger example:
CREATE OR REPLACE TRIGGER InventoryOthersMstrUpdate
BEFORE INSERT OR UPDATE  ON INVENTORY_OTHERS_MSTR
FOR EACH ROW
BEGIN
  IF (:NEW.TOTL_LEFT<>:OLD.TOTL_LEFT
     OR :NEW.TOTL_QTY<>:OLD.TOTL_QTY
  OR :NEW.TOTL_RESERVED<>:OLD.TOTL_RESERVED
  OR :NEW.HCARE_RESERVED_QTY<>:OLD.HCARE_RESERVED_QTY)
  THEN INSERT INTO INVENTORY_OTHERS_MSTR_TRACE (OTHERS_MSTR_ID, CATALOGUE_MSTR_ID, LAST_UPDATED_BY, LAST_UPDATED_ON, LAST_UPDATED_AT, CHECK_IND, LOCK_IND, LOC_IND, MAINTAIN_QTY, TOTL_QTY, TOTL_RESERVED, TOTL_LEFT, TOTL_LOAN, HCARE_RESERVED_QTY, LOC_ID)
  VALUES (:OLD.OTHERS_MSTR_ID, :OLD.CATALOGUE_MSTR_ID,
          :OLD.LAST_UPDATED_BY, :OLD.LAST_UPDATED_ON,
    :OLD.LAST_UPDATED_AT, :OLD.CHECK_IND, :OLD.LOCK_IND, :OLD.LOC_IND, :OLD.MAINTAIN_QTY, :OLD.TOTL_QTY, :OLD.TOTL_RESERVED, :OLD.TOTL_LEFT, :OLD.TOTL_LOAN, :OLD.HCARE_RESERVED_QTY, :OLD.LOC_ID);
  END IF;
END;
/

参考：http://www.chinaunix.net/jh/21/21696.html