Once the Virtuoso VDB Engine has been attached to an external data source, all objects (Tables, Views, Stored Procedures, etc.) accessible in that source may be exposed and addressed through a Virtual Schema in Virtuoso, as if they were local Virtuoso objects.
The general principles of this attachment are the same for all targets, but there are some known nuances to various targets. Source-specific articles covering these nuances are linked below.
Note: This feature is only available in the Commercial Editions (Personal, Enterprise, etc.) of Virtuoso.