Harold has kindly volunteered to lead the effort to create a SRFI specifying a Scheme API for talking to relational-style databases. I am very grateful to him for this: as chief cook and bottle-washer for the ongoing R7RS-large effort, I have more than enough on my plate without the database API too, and I was starting to feel "thin and stretched", as Bilbo says. I have bee impressed with Harold's effort and dedication on the SRFI 170 (Posix) design and implementation, and I am confident that he can produce a good result that will unify this very complex and difficult problem.
By "relational-style" I mean that the query language is not limited to SQL or even the general family of relational algebras. It can be anything at all, provided the result is organized as rows and named columns. Some graph query language provide this sort of output, and will be under the umbrella: others don't, and will require their own API. (Perhaps "rectangular" would be a better word than "relational" in "RDBMS".)
Harold tells me that he hopes to provide drivers for at least PostgreSQL, SQLite, and MySQL/MariaDB (the Widenius sisters), the most widely used open-source RDBMS engines. He will also be taking Oracle and Microsoft SQL Server, the most widely used proprietary engines, into account as far as possible, along with whatever else makes sense.
I look forward to seeing what he comes up with.