# [SOLVED] Simple way to calculate median with MySQL

## Issue

What’s the simplest (and hopefully not too slow) way to calculate the median with MySQL? I’ve used `AVG(x)` for finding the mean, but I’m having a hard time finding a simple way of calculating the median. For now, I’m returning all the rows to PHP, doing a sort, and then picking the middle row, but surely there must be some simple way of doing it in a single MySQL query.

Example data:

``````id | val
--------
1    4
2    7
3    2
4    2
5    9
6    8
7    3
``````

Sorting on `val` gives `2 2 3 4 7 8 9`, so the median should be `4`, versus `SELECT AVG(val)` which == `5`.

## Solution

``````SELECT AVG(dd.val) as median_val
FROM (
SELECT d.val, @rownum:[email protected]+1 as `row_number`, @total_rows:[email protected]
FROM data d, (SELECT @rownum:=0) r
WHERE d.val is NOT NULL
-- put some where clause here
ORDER BY d.val
) as dd
WHERE dd.row_number IN ( FLOOR((@total_rows+1)/2), FLOOR((@total_rows+2)/2) );
``````

Steve Cohen points out, that after the first pass, @rownum will contain the total number of rows. This can be used to determine the median, so no second pass or join is needed.

Also `AVG(dd.val)` and `dd.row_number IN(...)` is used to correctly produce a median when there are an even number of records. Reasoning:

``````SELECT FLOOR((3+1)/2),FLOOR((3+2)/2); -- when total_rows is 3, avg rows 2 and 2
SELECT FLOOR((4+1)/2),FLOOR((4+2)/2); -- when total_rows is 4, avg rows 2 and 3
``````