There was an SQL layer but performance was sub-par IIRC. There was also a blog post somewhere explaining why it's probably not a good idea to build an SQL layer on top of a KV store, devil in details, etc.

Edit: found it https://www.voltdb.com/blog/2015/04/foundationdbs-lesson-fas...

Not sure what to think of it, I'm not a DB expert by any means but the post sounds plausible enough and the SQL layer is discontinued AFAIK. I guess with each new abstraction layer you leave some perf on the table.