Get the final price with discount but only if column is not NULL. SQL

If you want to subtract only when your new column is not null, you could simply use IF() func inside you SUM()

In very simple example, assuming you added discount_code

create table Orders
(
  id int NOT NULL,
  price int NOT NULL,
  discount_code int NULL  
);

create table Discounts 
(
  id int not null,
  value int not null
);


insert into Orders
values
(1, 10, null),
(2, 10, null),
(3, 5, 1),
(4, 25, 1);


insert into Discounts
values
(1, 3);

select sum(if(o.discount_code is not null, o.price - d.value, o.price))
from Orders as o
left join Discounts as d
on o.discount_code = d.id;

-- 10 + 10 + 2 + 22 = 44

You can also run the example here