Hello,
I still don't know, what do you want to achieve. If you want to have Country Name in your custom table from another table, then the field can be defined through the same SQL query as you have already used, but then you need to fill its ID column also with the name of a country:
SELECT CountryName, CountryName FROM custom_table
If you want to have only an ID in your table, but you want to display a Country Name instead in frontend, then you can write your
own function in the custom table transformation, which takes an ID of your country and then returns its name.
Best regards,
Jan Hermann