That is a loaded question my friend and not really easy to answer (probably one reason no one has tried answering for a few months). It also depends on the perspective of the question (carrier, customer, data center, LAN, WAN,…).
In many respects SD-WAN is SDN (at least in its methodology), as its name implies, but not necessarily the other way around (a Square is a Rectangle, but a Rectangle is not a Square, by definition).
From my understanding, SDN simply separates the control plane, management plane, and data plane (“data plane” is a.k.a. forwarding plane, user plane, traffic plane, and maybe a few other “planes”). Where control & management are more centralized and then data is at the “edge”. Just like in SD-WAN, separating the control & management planes from the data plane opens up a new world of flexibility, agility, control, and capabilities.
While, IMHO, SD-WAN is a very practical and hugely beneficial solution for nearly any customer of nearly any size, SDN on the other hand is quite the opposite. While it may be quite useful and beneficial, it would be entirely impractical for nearly all small and mid-sized businesses, and even most large enterprises. I think even today, some of the largest (or smartest) carriers and other service providers are starting to realize its benefits and implemented a “full scale” SDN.
Just to be clear SDN is not really something that one can go a buy off the shelf. By that I mean there really aren’t switches or routers you by with ‘SDN included!’. It is more of a methodology, than a product itself.
(IMHO) probably one of the best ways to get to SDN is via SPB, or Shortest Path Bridging. This is a technology that is prevalent in many Data Centers and is very similar to other bridging tech that Carriers have been using for some time. Unfortunately many of the proprietary versions of this (Cisco, Juniper, Brocade, etc) limit their capabilities to relatively short distances therefore usually keeping it contained to within a smaller geo area or even a data center alone. SPB (also now an IEEE standard, 802.1aq) is specifically designed to extend to the network “edge”. Opening it up to a much larger audience.
“SPB allows customers to greatly simplify how they create and configure networks—across the enterprise and for the cloud—by requiring service provisioning only at the edge of the network. It uses Intermediate System To Intermediate System (IS-IS, unfortunate acronym, I know), a proven carrier-grade link state protocol, to dynamically build the topology between network nodes, saving network administrators time and effort, and virtually eliminating human error.” quoted from https://www.avaya.com/investors/usa/newsroom/news-releases/2011/pr-110907/
Sadly, Avaya recently sold off its entire data networking business to Extreme Networks. Hopefully Extreme sees the tremendous value in Avaya’s Fabric technology (SPB) and will continue to promote and develop it.
This is, coincidentally, the intended result of and for SDN. You might say that SPB smooths the way for SDN. Eventually making SDN readily available to the “common man”.
Also consider as things like IoT-Internet of Things continues its extremely rapid adoption and growth, SDN and SPB will become increasingly necessary to accommodate such enormous amounts of additional traffic and not just in data centers and carrier networks, but also into the business consumers LAN/WAN.
I hope this was helpful.