How to compare two databases without data access?

Question

Database

There are two databases. Databases contain personal data. Therefore, they cannot be accessed or compared directly. Is it possible to somehow compare 2 columns (for example, e-mail and a column with text, that is, this is no longer personal data) in one table with one column (e-mail) and show only the data of the column with text? At the same time, at no stage should there be access to the database of all e-mails

Answer 1

The question is somewhat similar to: "How to learn to ride a bike without a bike." It is not very clear how you are going to select data without data ...
Since there are no details, I will guess in the thick of it ...
1) As kmg4e rightly noted, you can compare the sum hash (what Google will tell you). Because Since the hash does not actually contain the data itself, you can query the hashes from the database. It's not really clear where you'll get the hash if you don't have access to the database...
2) In general, as a rule , the data in the database is divided into several tables. For example:
Table 1. Users: id, login, passHash, email, token , etc.
Table 2. Users_Profile: full name, age, address, blood type and other personal data .
Accordingly, it is possible at the SQL level to give the user access only to the first table and compare them. Here, as it were, the question is on what parameters you need to compare them.
3) Well nobody canceled Select with a choice of issued columns. You can make some kind of application with an API that (in JSON, for example, or immediately CSV) would issue only allowed data.

Answer 2

Compare hashes.

Answer 3

As an option: get access to a view with an "anonymized" data representation.
For example, simplified:
select md5(fio), md5(email) from table
and compare at least through not in