According to the docs as of this posting:
A Data Studio data source can connect to a single MySQL database table.
You can use multiple data sources (read: tables) in the same report, but you can't join them to use multiple data sources in one object, such as a bar graph or geo map. I was told by someone on the Data Studio team that joins are on their ...
You can actually add a joined tables to Google Data Studio - but it requires an extra step. In your mysql database, you must first create a view of the joined tables.
Here is a link that explains how to do that:
When you connect to your mysql datasource the joined view will ...
You partly answer your question. The feature is for "Report Level" in the same report. At the moment, you can have multiple pages follow the report level rule in the same report, but there is no feature to accommodate different reports.