Well, that's relationships between tables.. I'm talking complete different databases.
you think the concept is any different?
Referential Integrity has absolutely NOTHING to do with multiple databases.
From IBM...
Debbie
As noted, you can most certainly reference tables between databases, and if the databases are to relate in any specific way, you need to.
Please do not read a quick article on a certain subject and pretend that you know what you are talking about, you don't.
I was responding to a suggestion of using separate databases, which in this case would not benefit you in any sort of way.
To your question, encrypting data to be inserted into a db table is a waste of resources and db space.
Simply prevent SQL injection in the first place using the methods that you have already been given.