Q&A for professional and enthusiast programmers
SELECT 100*NRU.#NRU/#U FROM ( SELECT count(IU.Id) as '#U' FROM ( SELECT Users.Id as 'Id' FROM Users, Posts WHERE Users.Id = Posts.OwnerUserId and Posts.PostTypeId = 1 and Posts.Tags not like '%arduino%' GROUP BY Users.Id ) as IU ) as U, ( SELECT count(INRU.Id) as '#NRU' FROM ( SELECT Users.Id as 'Id' FROM Users, Posts WHERE Users.Id = Posts.OwnerUserId and Posts.PostTypeId = 1 and Posts.Tags not like '%arduino%' GROUP BY Users.Id HAVING count(Posts.Id) = 1 ) as INRU ) as NRU