instruction: Most prolific answering user for posts tagged with 'r'.
response: SELECT UserId, U.DisplayName, TagName, PostCount, RANK() OVER (PARTITION BY TagName ORDER BY PostCount DESC) AS PostCountRank FROM (SELECT COUNT(C.PostId) AS PostCount, C.UserId, C.UserDisplayName, T.TagName FROM Comments AS C INNER JOIN Posts AS P ON C.Id = P.AcceptedAnswerId INNER JOIN PostTags AS PT ON P.Id = PT.PostId INNER JOIN Tags AS T ON T.Id = PT.TagId WHERE T.TagName IN ('r', 'python', 'java', 'javascript', 'ruby', 'c++', 'c#') GROUP BY C.UserId, C.UserDisplayName, T.TagName) AS sub LEFT JOIN Users AS U ON UserId = U.Id ORDER BY PostCountRank LIMIT 70