DB/2. Not the best, but it's inbuilt, and accessible with system utilities/calls, using any language on the system, including control language.
Say what you will but DB/2 on IBM i is the most SQL compliant database of any. It scales well and being built into the system means tuning it is not often needed. In addition there is near zero database administrative effort required for most IBM i systems. Since all disk space is typically viewed as a single store, space for tables and collections is automatically drawn from and returned to that space.
If you can't understand it, it is intuitively obvious.