Formally, a database URI as defined here is an opaque URI starting with db: followed by an embedded server-style URI. For example, this database URI:
Is formally the URI pg://localhost/mydb embedded in an opaque db: URI. It adheres to this formal definition because the scheme part of a URI is not allowed to contain a sub-scheme (or subprotocol, in the JDBC parlance). It is therefore a legal URI embedded in a second legal URI.