create table sales_orders (
  id int primary key,
  ...
);

create table order_items (
  order_id int references sales_orders(id),
  line_number smallint,
  unit_price decimal(19,4) not null,
  quantity smallint not null default 1,
  product_id int not null references products(id),
  ...

  primary key (order_id, line_number)
);