Average number of members' replies per topic (excluding staff)

For some strange reason I know that id of staff is three. But how could one find that id? First I was totally sure that is baked in urls, like every? other ids, but no. There is only name in use.

I know so little SQL that we can easily say I can’t, but this shows every group-id

select 
    id, 
    name
from 
    groups

But surely there is more common way to find it, isn’t there?