問(wèn)題描述

使用 MySQL 計(jì)算中位數(shù)的最簡(jiǎn)單(希望不會(huì)太慢)方法是什么?我已經(jīng)使用 AVG(x) 來(lái)找到平均值，但我很難找到一種計(jì)算中位數(shù)的簡(jiǎn)單方法.現(xiàn)在，我將所有行返回給 PHP，進(jìn)行排序，然后選擇中間的行，但肯定有一些簡(jiǎn)單的方法可以在單個(gè) MySQL 查詢(xún)中執(zhí)行此操作.

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.

示例數(shù)據(jù):

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

對(duì) val 排序給出 2 2 3 4 7 8 9，所以中位數(shù)應(yīng)該是 4，而 SELECT AVG(val) which == 5.

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

推薦答案

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 將包含總行數(shù).這可用于確定中位數(shù)，因此不需要第二遍或連接.

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.

還有 AVG(dd.val) 和 dd.row_number IN(...) 用于在記錄數(shù)為偶數(shù)時(shí)正確生成中位數(shù).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+ 包含一個(gè) MEDIAN 函數(shù)

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