用MySQL计算中位数最简单(希望不会太慢)的方法是什么?我已经使用AVG(x)来寻找平均值，但我很难找到一个简单的方法来计算中位数。现在，我将所有的行返回到PHP，进行排序，然后选择中间的行，但是肯定有一些简单的方法可以在一个MySQL查询中完成它。
示例数据:
id | val
--------
1 4
2 7
3 2
4 2
5 9
6 8
7 3
对val排序得到2 2 3 4 7 8 9，因此中位数应该是4，而SELECT AVG(val) == 5。
当前回答
您可以使用窗口函数row_number()来回答查询以找到介质
select val
from (select val, row_number() over (order by val) as rownumber, x.cnt
from data, (select count(*) as cnt from data) x) abc
where rownumber=ceil(cnt/2);
其他回答
SELECT
SUBSTRING_INDEX(
SUBSTRING_INDEX(
GROUP_CONCAT(field ORDER BY field),
',',
((
ROUND(
LENGTH(GROUP_CONCAT(field)) -
LENGTH(
REPLACE(
GROUP_CONCAT(field),
',',
''
)
)
) / 2) + 1
)),
',',
-1
)
FROM
table
上面的方法似乎对我有用。
这是我的办法。当然，你可以把它放到一个过程中:-)
SET @median_counter = (SELECT FLOOR(COUNT(*)/2) - 1 AS `median_counter` FROM `data`);
SET @median = CONCAT('SELECT `val` FROM `data` ORDER BY `val` LIMIT ', @median_counter, ', 1');
PREPARE median FROM @median;
EXECUTE median;
你可以避免变量@median_counter，如果你替换它:
SET @median = CONCAT( 'SELECT `val` FROM `data` ORDER BY `val` LIMIT ',
(SELECT FLOOR(COUNT(*)/2) - 1 AS `median_counter` FROM `data`),
', 1'
);
PREPARE median FROM @median;
EXECUTE median;
下面的SQL代码将帮助您使用用户定义的变量来计算MySQL中的中位数。
create table employees(salary int); insert into employees values(8); insert into employees values(23); insert into employees values(45); insert into employees values(123); insert into employees values(93); insert into employees values(2342); insert into employees values(2238); select * from employees; Select salary from employees order by salary; set @rowid=0; set @cnt=(select count(*) from employees); set @middle_no=ceil(@cnt/2); set @odd_even=null; select AVG(salary) from (select salary,@rowid:=@rowid+1 as rid, (CASE WHEN(mod(@cnt,2)=0) THEN @odd_even:=1 ELSE @odd_even:=0 END) as odd_even_status from employees order by salary) as tbl where tbl.rid=@middle_no or tbl.rid=(@middle_no+@odd_even);
如果你正在寻找详细的解释，请参考这个博客。
set @r = 0;
select
case when mod(c,2)=0 then round(sum(lat_N),4)
else round(sum(lat_N)/2,4)
end as Med
from
(select lat_N, @r := @r+1, @r as id from station order by lat_N) A
cross join
(select (count(1)+1)/2 as c from station) B
where id >= floor(c) and id <=ceil(c)
安装和使用本mysql统计函数:http://www.xarg.org/2012/07/statistical-functions-in-mysql/
之后，计算中值就很简单了:
SELECT median(val) FROM data;
