microsoft access training - Relationships between tables?

If I want to link two tables do they need to have a unique key that they share or would a matching address field i.e. postcode be enough?

I have a database that my company uses. It records the contact details of our customers. We also purchase a list of UK Businesses. I would like to add information from that list to our database. However due to us using a different type of recording ID's to the purchased list, the only thing they may share is address details such as postcode.

Can the postcode be the relationship?

Thanks