Oracle Trigger on two tables - oracle

Question

I am trying to create a BEFORE UPDATE trigger on the two tables purchaseorderdetail and purchaseorderheader as shown below. I would like to implement this trigger on PurchaseOrderHeader table that prohibits updates of the PurchaseOrderHeader.SubTotal column if the corresponding data in the PurchaseOrderDetail table is not consistent with the new value of the PurchaseOrderHeader.SubTotal column.
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)
);
What I have come up with so far, any help?
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;

Answer 1

Compute total separately (and reference the master key), and then compare that value to new subtotal.
CREATE OR REPLACE TRIGGER Header_Before_Subtotal
BEFORE UPDATE
ON purchaseorderheader
FOR EACH ROW
DECLARE
l_total number;
BEGIN
SELECT d.unitprice * d.orderqty
INTO l_total
FROM purchaseorderdetail d
WHERE d.purchaseorderid = :new.purchaseorderid;
IF :NEW.subtotal <> l_total THEN
RAISE_APPLICATION_ERROR(-20001, 'Subtotal is not equal to unitprice * order quantity, check again.');
END IF;
END;
/