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select t.TagName, count(t.TagName) as numberOfQuestion from Posts pos join (select p.Id, count(p.Id) as tagsNum from PostTags pt join Posts p on pt.PostId = p.Id group by p.Id) tab on tab.Id = pos.id join PostTags pt on pt.PostId = pos.Id join Tags t on pt.TagId = t.Id where tab.tagsNum = 1 group by t.TagName order by numberOfQuestion desc