How to merge user tables from 2 different projects?

Question

SQL

There are 2 projects with tables identical in structure. Users are added to them with their login, email and password. Moreover, there are options that one user participates in both projects. Sometime under the same logins, sometime under different ones. It happens that one login is busy in another project and another one is taken. Some users are in only one of the projects, etc.
The task was to combine everything within the framework of one project, which means it is necessary to merge users into one table. How to do it in the least traumatic way for them? It is clear that you can add a project prefix to the login, for example. But I want to do it more gracefully, for example, merge 2 users from different projects into one if they have a common owner.

Answer 1

Reading about Identity Server and User Pool