Get number of users logged in via mySQL

[Renato Sousa]

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