Google-sheets – Insert Text from Spreadsheets to Data Studio

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 roadmap.

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.