What tools have people used to build servers that speak SQL? Where SQL becomes an API protocol like GraphQL, whether it talks to a database or not doesn't matter.
I worked for a company (startup) that built a very unique engine. It was SQL on top of anything - literally.
It fell into the domain of "data federation" or "data virtualization" - something that is quite overlooked by the new generation of tools such as GraphQL - but they are starting to realize that federation/virtualization of data is exactly what these tools provide. One need look no further than Apollo GraphQL to find language showing they are starting to understand it better.
Anyway, Calcite is great for building a SQL engine (parser, lexer, planner, optimizer) because it's a framework independent of any backend system and storage engine.
That said, other people have taken the approach of using any open source SQL database, including Postgres, Apache Derby, SQLite - and basically tearing out the SQL engine, wire protocols, JDBC drivers, and removing the storage engine piece and making that pluggable.
As some others in the post have mentioned, SQL/MED is a feature built in to a number of databases, but if you really wanted just the SQL engine without the rest of the DBMS, the approach above is the one I have seen - deconstruct the database into just the SQL engine and just use that.
These days you can also try to use things like Apache Spark as a programming framework to build such an abstraction as well (with dataframes and SQL over those).
Something like https://calcite.apache.org/?
(It's hard to google for sql server, because you know...)