Digital Equipment Corporation sold a SQL database known as Rdb that could also run as a single file.
It was the first database to introduce a cost-based optimizer, and ran under both VMS and Digital UNIX.
Oracle bought it, and VMS versions are still supported.
https://www.oracle.com/database/technologies/related/rdb.htm...
https://en.m.wikipedia.org/wiki/Oracle_Rdb
(My employer is still using the VMS version.)
Oh! RDB was the first database I worked with. I forgot all about it. I do remember refactoring the data layer so that it also worked with Berkeley DB, which is also owned by Oracle now. Or maybe it was the other way around? There was no SQL involved in that particular application so it was just a K/V store. Working with a local data file was the primary design goal, no client/server stuff was even on the radar. SQLite would have been perfect if it had existed.