Any DBMS can be run "in memory". Consider using ramdisk . However, most DBMSs (with SQL) are not designed to work fully in memory and put a lot of effort into minimizing disk I / O and swap: the DBMS works very hard so that the "relevant data" is hot (in memory and in cache) - - IO slow, slow slow.
This is because database data is often [and historically] significantly larger than main memory. This and main memory are unstable :-) [ACID DBMSs do a lot of work with future writes) to non-volatile storage - and other methods that ensure that data is never corrupted, even in the event of an unexpected shutdown. ]
Some databases, such as SQLite, use the same format for disk and storage devices, although they explicitly support in-memory storage. Support for other [in-memory] back-end and memory usage settings is vendor-specific.
Happy coding.
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