pl/sql statement Trigger - oracle

Question

Suppose we have the following two tables:
OrderHeader(OrderID, Odate, CustID, Total)
Order_Item(OrderID,ItemID, Qty, Subtotal)
Write a statement-level trigger that updates the Total in the orderHeader table with
the total value of the order_item records whenever an insert, update or delete event
occurs on the order_item table. For any update error, raise an exception.
So far I have written this:
create or replace TRIGGER SECURE_ORDER
AFTER INSERT OR DELETE OR UPDATE ON Order_Item
declare
sum1 Number;
BEGIN
SELECT SUM(Qty*Price) Into sum1 FROM Order_Item Inner join Item Using(ItemID) WHERE OrderID=:OLD.OrderID;
UPDATE OrderHeader set Total=sum1 where OrderID=:OLD.OrderID;
EXCEPTION
WHEN too_many_rows then
dbms_output.put_line('Too many rows');
WHEN no_data_found then
dbms_output.put_line('No Data Found');
WHEN others then
dbms_output.put_line('other error');
END;
It is not correct however as the question statement says I have to write a statement level trigger which doesn't give me acces to NEW and OLD keyword. I don't know how to go about doing this in a statement level trigger. Any help would be appreciated.

Answer 1

You can just use a single MERGE statement:
CREATE TRIGGER SECURE_ORDER
AFTER INSERT OR DELETE OR UPDATE ON Order_Item
BEGIN
MERGE INTO OrderHeader dst
USING (
SELECT COALESCE( oh.OrderID, o.OrderID ) AS OrderID,
COALESCE( SUM( o.Qty*i.Price ), 0 ) AS total
FROM OrderHeader oh
FULL OUTER JOIN Order_Item o
ON oh.OrderID = o.OrderID
LEFT OUTER JOIN Item i
ON ( o.ItemID = i.ItemID )
GROUP BY COALESCE( oh.OrderID, o.OrderID )
) src
ON ( src.OrderID = dst.OrderID )
WHEN MATCHED THEN
UPDATE SET total = src.total
WHEN NOT MATCHED THEN
INSERT ( OrderID, Total ) VALUES ( src.OrderID, src.Total );
END;
/
Then, if you have the tables:
CREATE TABLE Item (
ItemID NUMBER(10,0)
GENERATED ALWAYS AS IDENTITY
CONSTRAINT Item__ItemID__PK PRIMARY KEY,
Name VARCHAR2(20)
NOT NULL,
Price NUMBER(12,2)
NOT NULL
CONSTRAINT Item__Price_GTE_0__CHK CHECK ( Price >= 0 )
);
CREATE TABLE Order_Item (
OrderID INT
-- CONSTRAINT Order_Item__OrderID__FK REFERENCES Orders ( OrderID )
NOT NULL,
Qty INT
NOT NULL
CONSTRAINT Order_Item__Qty_GT_0__CHK CHECK ( Qty > 0 ),
ItemID INT
NOT NULL
CONSTRAINT Order_Item__ItemID__FK REFERENCES Item ( ItemId ),
CONSTRAINT Order_Item__OID__IID__PK PRIMARY KEY ( OrderID, ItemID )
);
CREATE TABLE OrderHeader (
OrderID INT
NOT NULL,
total NUMBER(14,2)
NOT NULL
);
Then if you:
INSERT INTO Item ( Name, Price ) VALUES ( 'Item 001', 1.00 );
INSERT INTO Item ( Name, Price ) VALUES ( 'Item 002', 2.00 );
INSERT INTO Item ( Name, Price ) VALUES ( 'Item 003', 2.50 );
INSERT INTO Order_Item ( OrderID, ItemID, Qty ) VALUES ( 1, 1, 3 );
INSERT INTO Order_Item ( OrderID, ItemID, Qty ) VALUES ( 1, 2, 1 );
INSERT INTO Order_Item ( OrderID, ItemID, Qty ) VALUES ( 1, 3, 2 );
The OrderHeader table contains:
SELECT * FROM OrderHeader;
ORDERID | TOTAL
------: | ----:
1 | 10
Then if you do:
UPDATE Order_Item
SET Qty = 8
WHERE OrderID = 1
AND ItemID = 1;
The OrderHeader table contains:
SELECT * FROM OrderHeader;
ORDERID | TOTAL
------: | ----:
1 | 15
Then if you do:
DELETE FROM Order_Item
WHERE ( OrderID, ItemID ) IN ( (1,2), (1,3) );
The OrderHeader table contains:
SELECT * FROM OrderHeader;
ORDERID | TOTAL
------: | ----:
1 | 8
And, finally, if you do:
DELETE FROM Order_Item;
The OrderHeader table contains:
SELECT * FROM OrderHeader;
ORDERID | TOTAL
------: | ----:
1 | 0
db<>fiddle here