is it possible to install mikrotike database server, such as mySQL server? or other types of databases

can you explain a bit

and if you want to Install database server on mikrotik router i guess you can't
and if you want to use mikrotik as hostspot/ppp server and want the users list from database then you need to configure Radius

It's a router, not server. But if you do need it badly, metarouter is your only chance of running sql server on RB hardware.