Use the database connector https://appstore.home.mendix.com/link/app/2888/ to import the data from the oracle DB into the Mendix app/DB, Then Mendix will take care of the ID generation and joining tables. Don't try to manage that your self
See my answer on how the Mendix ID is being constructed. I see two problems:
Not sure what you really are trying to achieve but Mendix can actually migrate from one database type (say MS SQL) to another (Oracle)? See:
https://docs.mendix.com/howto/data-models/migrating-your-mendix-database