Where "active" is defined as has answered a question within the last 12 months, regardless of whether the answer was accepted or upvoted. Note that this doesn't count users whose answers have been deleted.
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-- Count all Posts that are:
-- 1. Answers
-- 2. Have a distinct OwnerUserId
-- 3. Have a CreationDate after the date of 1 year ago today
COUNT(DISTINCT OwnerUserId) AS 'Active Users'
PostTypeId = 2
--AND CreationDate > DATEADD(year, -1, GETDATE())
AND CreationDate > '##StartTime?2019/01/12 00:00:00##'
--AND CreationDate <= DATEADD(year, 1, '##StartTime##')
-- Get total users for comparison
COUNT(Id) AS 'Total Users'