How to combine aggregate functions in MySQL?

Question

I'm just learning MySQL - is there a way to combine (or nest) aggregate functions?

Given a query:

SELECT user, count(answer) FROM surveyValues WHERE study='a1' GROUP BY user;

This will give me the number of questions answered by each user. What I really want is the average number of questions answered per user...something like:

SELECT avg(count(answer)) FROM surveyValues WHERE study='a1';

What's the correct way to compute this statistic?

If this is possible, is there a way to then break this statistic down for each question? (users can answer the same question multiple times). Something like:

SELECT avg(count(answer)) FROM surveyValues WHERE study='a1' GROUP BY question;

Answer 1

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...