用MySQL计算中位数的简单方法

问题描述

用MySQL计算中位数的最简单方法(希望不是太慢)是什么?我已经使用AVG(x)来找到均值，但是我很难找到一种简单的方法来计算中位数.现在，我将所有行返回给PHP，进行排序，然后选择中间行，但是肯定必须有一些简单的方法可以在单个MySQL查询中完成.

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.

示例数据:

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

对val进行排序得到2 2 3 4 7 8 9，因此中位数应为4，而对SELECT AVG(val)的中位数应为== 5.

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

推荐答案

在MariaDB/MySQL中:

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 指出，在第一遍之后，@ rownum将包含总行数.可以用来确定中位数，因此不需要第二次通过或联接.

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.

也AVG(dd.val)和dd.row_number IN(...)用于正确产生中位数.推理:

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

最后， MariaDB 10.3.3+包含MEDIAN函数

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