instruction: Answerers per Questioners by month for tag since 2010.
response: SELECT DATEFROMPARTS(YEAR(pq.CreationDate), MONTH(pq.CreationDate), 1) AS ym, CAST(COUNT(DISTINCT CASE WHEN pa.PostTypeId = 2 AND pa.Score > 0 THEN pa.OwnerUserId ELSE NULL END) AS FLOAT) / COUNT(DISTINCT CASE WHEN pq.PostTypeId = 1 AND pq.Score > 0 THEN pq.OwnerUserId ELSE NULL END) AS a_per_q FROM Posts AS pq LEFT JOIN Posts AS pa ON pq.Id = pa.ParentId JOIN PostTags AS pt ON pq.Id = pt.PostId JOIN Tags AS t ON t.Id = pt.TagId WHERE t.TagName = '##Parameter##' AND YEAR(pq.CreationDate) >= 2010 GROUP BY DATEFROMPARTS(YEAR(pq.CreationDate), MONTH(pq.CreationDate), 1) ORDER BY DATEFROMPARTS(YEAR(pq.CreationDate), MONTH(pq.CreationDate), 1)