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 WHER         


        

Answer 1

yes - those all look reasonable.

Have you tried them and received unexpected results?

usually, i would expect that you also include the driving column in the select list:

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

Answer 2

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...