Readers who like SQL may also enjoy Steampipe [1], an open source tool to live query 99+ services with SQL (e.g. AWS, GitHub, CSV, Kubernetes, etc). It uses Postgres Foreign Data Wrappers under the hood and supports joins etc across the services. (Disclaimer - I'm a lead on the project.)
Correct - it's just Postgres, so all your favorite clients / tools work as usual. We have docs for a number of example integrations [1] that work with Steampipe CLI (or cloud).
Steampipe runs an embedded Postgres database [1]. One approach would be to connect that Postgres database with other databases using the usual (advanced) techniques like Foreign Data Wrappers [2]? With that connection in place, joins would work as normal.
1 - https://github.com/turbot/steampipe