在MariaDB/MySQL中:
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点指出,在第一次传递之后,@rownum将包含总行数.这可以用来确定中间值,因此不需要第二次通过或连接.
当记录数为偶数时,AVG(dd.val)
和dd.row_number IN(...)
也用于正确生成中值.推理:
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
最后,MariaDB 10.3.3+ contains a MEDIAN function