I have another post looking at external columns on PolyBase V2 data sources:
I’m going to use external two tables in this experiment. In the left corner, we have some ORC files stored in Azure Blob Storage which we’ll represent as FireIncidents2017. In the right corner, we have data stored in a remote SQL Server instance which we’ll call LineItem. The data doesn’t really matter that much, but to give you an idea of where we’re going, I’ll show each table.
There’s quite a bit you can do here.