I've used ports of BerkleyDB several times for things down at that level. It isn't very powerful, but it's enough you can use it for most microcontroller needs.

http://iondb.org/ is one.

Putting SQLite onto a controller that isn't running a full OS is actually a lot of work. [0] There isn't a working RTOS port out there, last I checked.

[0] https://sqlite.org/custombuild.html

Well yeah, depends on the boards limitations.