SQL error - trigger/function may not see it - oracle

Question

I was working on my project, here I got this error while inserting some values in a row:
ERROR at line 1: ORA-04091: table SYSTEM.PRODUCTS is mutating,
trigger/function may not see it ORA-06512: at "SYSTEM.PROD_TOTAL",
line 2 ORA-04088: error during execution of trigger
'SYSTEM.PROD_TOTAL'
This is my insert statement:
insert into products
values (1, 1001, 'Medical', 20, 4, 1, 1, 1);
Products table :
create table Products
(
ProdId number primary key,
ProdNum number not null unique,
ProdType varchar2(15),
ProdPrice int,
ProdQuantity int,
ProdCustId int references Customers,
ProdOrdId int references Orders,
ProdStoreId int references Stores
);
Trigger code:
create trigger PROD_TOTAL
after insert ON Products
for each row
begin
update Payments
set ProdTotal = (select Products.ProdPrice * Products.ProdQuantity from Products);
end;
/
And finally my Payment table:
create table Payments
(
PayId int primary key,
PayDate date,
ProdTotal int,
FinalTotal int,
PayOrdId int references orders,
PayProdId int references Products,
PayCustId int references Customers
);
I don't know why I am getting this error, please help me in solving this issue...

Answer 1

A statement level trigger (i.e. without FOR EACH ROW clause) will update always all records in Payments table, I don't think that's needed. For an update of only related products, use this trigger:
create trigger PROD_TOTAL
after insert ON Products
for each row
begin
update Payments
set ProdTotal = :new.ProdPrice * :new.ProdQuantity
WHERE PayProdId = :new.ProdId ;
end;