A user is considered "active" if they have > "threshhold" posts in the given set of tags, where "threshhold" is a parameter that can be set by modifying the textbox below.
Q&A about the site for linguists, teachers and students of the Ukrainian language
select Z.[Month], count(Z.ouid) cnt, Z.ouid as [User Link]
cast(DATEPART(YYYY, Y.CreationDate) as varchar) + '-' + cast(DATEPART(MM, Y.CreationDate) as varchar) + '-01'
as datetime) [Month], ouid
select p1.creationdate CreationDate ,p1.owneruserid ouid from posts p1
inner join posts p2 on p1.parentid=p2.id
where p1.posttypeid=2 and
(p2.tags LIKE '%matlab%')
group by p1.owneruserid, p1.creationdate
) Y group by ouid, CreationDate
) Z group by Z.ouid,Z.[Month]
) as X where X.cnt>##Threshhold?2##
group by [Month]
order by [Month] asc