Microsoft working on a file system that is SQL Server based. It's called WinFS. At one time, it was to be part of Longhorn, but has been pulled, presumably because it still has lots of problems. It's not the entire OS, just the file system. I don't think you could have all the OS services bases on SQL Server.

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>I heard something about Microsoft wanting to have a future file-system on the basis of a DBMS (SQL Server, of course). But I don't know much details about this. - I think it would be only the file system, not other parts of the OS, which might be based on the DBMS.