我正尝试在两个表purchaseorderdetail和purchaseorderheader上创建BEFORE UPDATE触发器，如下所示。我希望在PurchaseOrderHeader表上实现此触发器，如果PurchaseOrderDetail表中的相应数据与PurchaseOrderHeader.SubTotal列的新值不一致，则禁止更新PurchaseOrderHeader.SubTotal列。

CREATE TABLE purchaseorderheader(
purchaseorderid NUMBER(4),
revisionnumber NUMBER(2),
status NUMBER(1),
employeeid NUMBER(3),
vendorid NUMBER(4),
shipmethodid NUMBER(1),
orderdate TIMESTAMP,
shipdate TIMESTAMP,
subtotal FLOAT(10),
taxamt FLOAT(10),
freight FLOAT(10),
modifieddate TIMESTAMP,
PRIMARY KEY(purchaseorderid)
);

CREATE TABLE purchaseorderdetail(
purchaseorderid NUMBER(4),
purchaseorderdetailid NUMBER(4),
duedate TIMESTAMP,
orderqty NUMBER(6),
productid NUMBER(6),
unitprice FLOAT(10),
receivedqty FLOAT(10),
rejectedqty FLOAT(10),
modifieddate TIMESTAMP,
PRIMARY KEY(purchaseorderdetailid),
CONSTRAINT fk_orderid FOREIGN KEY (purchaseorderid) REFERENCES purchaseorderheader(purchaseorderid)
);

到目前为止我所想到的，有帮助吗？

CREATE OR REPLACE TRIGGER Header_Before_Subtotal BEFORE UPDATE  
ON purchaseorderheader 
FOR EACH ROW
BEGIN
    IF(:NEW.subtotal <> (SELECT unitprice*orderqty FROM purchaseorderdetail GROUP BY purchaseorderid)) THEN
        RAISE_APPLICATION_ERROR(-20001, 'Subtotal is not equal to unitprice * order quantity, check again.');
    END IF;
END;