How to combine aggregate functions in MySQL?

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You have to use subqueries:

  SELECT x.user, 
         AVG(x.cnt)
    FROM (SELECT user, COUNT(answer) AS cnt
            FROM surveyValues 
           WHERE study='a1' 
        GROUP BY user) x
GROUP BY x.user

You can’t wrap an aggregate with another aggregate. You could wrap an analytic in an aggregate, if MySQL supported analytic/ranking/windowing functions…

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