Defining a primary key: Customer numbers are unique only within a country but may be duplicated across different countries I am facing the following problem:

I have a table with customer information like the customer number, i.e. customerID set up as the primary key, and, besides a number of other data, also a country value in the form of a country code, countryID.

However, customer data from other countries should also be imported into this table. Although the customer numbers are unique within a country, they are not cross-country. Customer numbers should not be changed for organizational reasons.

How can a key still be ensured for each customer in this table?
I would use countryID, CustomerID as your unique key as you have stated that you will have a unique customer id for each country. I would start with countryID as that will be unique in the system and you can add in customerID in order to get a unique key for each customer.