If you need full-time synchronization between an existing Notes infrastructure and a RDBMS, LEI (Lotus Enterprise Integrator) or a third-party tool like Notrix would be your best bet -- it's as simple as defining a job and a schedule/trigger to run it. If you need to occasionally pull (or push) a subset of the data, then NotesSQL is probably the easiest approach. If you're not afraid of learning the structure of the NSF (Notes Storage Facility), then the LotusScript/COM API or the Java/CORBA API would give you finer-grained control.
If what you really need is a one-time dump of everything, then exporting all of the data notes to DXL (Domino XML) would give you the most complete version of the data you're going to get, and in a way that would let you recover and convert formatted Notes Rich Text, file attachments, and so on in a way that would be incredibly difficult to achieve otherwise. DXL is verbose, so don't say I didn't warn you, but it is pretty comprehensive as well. (The DOmino Designer Help entry on the NotesDXLExporter class has example code that is exactly on point.)