I just found another answer online in the comments:

Make sure your columns are well indexed and that the index is used for filtering and sorting. Validate through execution plan.

select count(*) from table --找到行数

Calculate the "median" row number. Possible use: median_row = floor(count / 2).

Then select from the list:

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

This should return a value you want.

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
  -- 在这里放置一些条件语句
  ORDER BY d.val
) as dd
WHERE dd.row_number IN ( FLOOR((@total_rows+1)/2), FLOOR((@total_rows+2)/2) );

Steve Cohen pointed 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 required.

Additionally, AVG(dd.val) and dd.row_number IN(...) are used to correctly calculate the median when the number of records is even. The reasoning is as follows:

SELECT FLOOR((3+1)/2),FLOOR((3+2)/2); -- 当total_rows为3时，平均行数为2和2
SELECT FLOOR((4+1)/2),FLOOR((4+2)/2); -- 当total_rows为4时，平均行数为2和3

Finally, MariaDB 10.3.3 includes the MEDIAN function