Get number of users logged in via mySQL

Hello All, I would like to get the number of logged in users on freeradius server (3.0.12). Looking at the mySQL accounting table, the connections with the NULL value for the acctstoptime field, I see some connections with the same user and even device mac-address with the NULL acctstoptime field. How to get this precise value through mySQL tables? How to delete these open sessions? Best Regards