What you have is a perfect example for one-to-many relationship where you have one order and several items attached to it. You should have a link table like
order_product - which makes the connection between a orderid and productid where you can also put specific data for the relationship between the two (like when the item was added, quantity, etc)
Then you make the join using this table and you have same field types everywhere.
simple example:
select
/* list of products */
from
order o,
order_product op,
product p
where
o.id = 20
and o.id = op.orderid
and op.productid = p.id