Q&A for professional mathematicians
select eomonth(CreationDate) [month], COUNT(DISTINCT UserId), COUNT(DISTINCT UserDisplayName), COUNT(DISTINCT UserId)+COUNT(DISTINCT UserDisplayname) FROM Comments GROUP BY eomonth(CreationDate) ORDER BY eomonth(CreationDate) --UserDisplayname is shown when user account was deleted since then --It is included to count also users that have been deleted in the meantime