MySQL query to get the modal averages of a column?

Question

I\'ve been recording the window height of users of my control panel.

I have a table like so:

user_id(INT) | window_height(INT)
--------------------------         


        

Answer 1

To get raw counts

select window_height, count(*) totalusers
from tbl
group by window_height
order by totalusers desc  # or by window_height

To get the modal average (this will show multiple values if there are ties for the highest count)

select window_height, totalusers
from (
    select @r := if(totalusers>@r,totalusers,@r) maxcount, window_height, totalusers
    from (select @r:=0) initvars, (
        select window_height, count(*) totalusers
        from tbl
        group by window_height
    ) X ) Y
where totalusers = @r

This uses a MySQL trick of using a variable to store the max count as it goes through the aggregated subquery. Summary of operations

• O(n): scan table once and build the counts (T1)
• O(n): scan the derived table T1 and keep the highest count in the variable @r (T2)
• O(n): scan the derived table T2 and filter only for the heights with the highest count