Sql 编写异常处理程序

我正在尝试创建一个异常处理程序，以防止插入空值。我还想将它设置为，如果插入的是空值，则原始成本将显示在消息中

我的两张桌子是

create table pitem,
(item_id  number,
item_cost  number(10,2)
);

和

这是程序

create or replace procedure update_item_cost
(iItemId INTEGER,
fNewcost NUMBER) AS
fCurCost NUMBER(10,2);
missing_cost EXCEPTION;
BEGIN
   select item_cost INTO fCurCost from pitem
   where item_id=iItemId;
   IF fCurCost IS null THEN
   RAISE missing_cost;
   ELSE
   UPDATE pitem SET item_cost=fNewcost
      where item_id=iItemId;
   END IF;
COMMIT;
EXCEPTION
  WHEN NO_DATA_FOUND THEN
     INSERT INTO pitem_audit VALUES (iItemId, 'Invalid Item identifier.');
     COMMIT;
  WHEN missing_cost THEN
     INSERT INTO pitem_audit VALUES (iItemId, 'Item cost is null.');/*<---heres where I want it to use the original cost of the item saying 'Null cost replaced by original cost of whatever the orignal was*\
     COMMIT;
  WHEN OTHERS THEN
     ROLLBACK;
     INSERT INTO pitem_audit VALUES (iItemId, 'Miscellaneous error.');
     COMMIT;
  END update_item_cost;
/

我的问题是如何编写异常处理程序来防止输入空值..我应该只执行ALTER TABLE语句并添加NOTNULL约束吗？另外，我如何获得pitem_审计表下的消息以显示“Null cost替换为[original item_cost]的原始值”

---

提前感谢：D

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create or replace procedure update_item_cost
(iItemId INTEGER,
fNewcost NUMBER) AS
fCurCost NUMBER(10,2);
missing_cost EXCEPTION;
BEGIN
   select item_cost INTO fCurCost from pitem
   where item_id=iItemId;
   IF fCurCost IS null THEN
   RAISE missing_cost;
   ELSE
   UPDATE pitem SET item_cost=fNewcost
      where item_id=iItemId;
   END IF;
COMMIT;
EXCEPTION
  WHEN NO_DATA_FOUND THEN
     INSERT INTO pitem_audit VALUES (iItemId, 'Invalid Item identifier.');
     COMMIT;
  WHEN missing_cost THEN
     INSERT INTO pitem_audit VALUES (iItemId, 'Item cost is null.');/*<---heres where I want it to use the original cost of the item saying 'Null cost replaced by original cost of whatever the orignal was*\
     COMMIT;
  WHEN OTHERS THEN
     ROLLBACK;
     INSERT INTO pitem_audit VALUES (iItemId, 'Miscellaneous error.');
     COMMIT;
  END update_item_cost;
/

DECLARE
 item_ident number;
 cost number;
BEGIN
 item_ident := 10;
 cost :='';
 update_item_cost(item_ident, cost);
END;
/