In MariaDB / MySQL:

SELECT AVG(dd.val) as median_val

FROM (

SELECT d.val, @rownum:=@rownum+1 as `row_number`, @total_rows:=@rownum

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

Finally, MariaDB 10.3.3+ contains a MEDIAN function

I just found another answer online in the comments:

For medians in almost any SQL:

SELECT x.val from data x, data y

GROUP BY x.val

HAVING SUM(SIGN(1-SIGN(y.val-x.val))) = (COUNT(*)+1)/2

Make sure your columns are well indexed and the index is used for filtering and sorting. Verify with the explain plans.

select count(*) from table –find the number of rows

Calculate the “median” row number. Maybe use: median_row = floor(count / 2).

Then pick it out of the list:

select val from table order by val asc limit median_row,1

This should return you one row with just the value you want.

Jacob