Yes, you can certainly couple C2 with SQL Server, by using Construct's AJAX object to make a request to a server-side script (e.g. a .NET .ashx handler) that calls a stored procedure in the database. I've done this to create a multiplayer "simcity"-style game in a shared world, but never an MMORPG.
Whether this is a good design for your game or not really depends on the requirements of your project. SQL Server is a relational database, just like any other (Postgres, MySQL, Oracle, DB2...). It's good for storing and accessing large amounts of data (think gigabytes at the minimum), but it's not going to be as efficient as an in-memory or local storage option if you're just storing a small amount of data.
If you're going to be loading level/terrain data in discrete stages (e.g. retrieve the current location of items/characters from the database when the game first loads, or when the player enters a new area, and write back to the database only occasionally when they change) then this would work well. However, it would not be so efficient if you want to send continuous packets of data to and from the server representing each character's actions in real-time.
As others have mentioned, there are a lot of complex issues in MMORPG design, and a lot of clever workarounds required to keep the world synchronised between players. If this is your first project, I'd seriously recommend you scale down your aspirations to start with - perhaps just create a 2-player game in which the world is synchronised via a central server - and then work up to something like this. Good luck!