mysql-求中位数方法

mysql中并没有直接求中位数的函数，经过一番查找之后，找到了如下方法：

SELECT 
((SUBSTRING_INDEX(SUBSTRING_INDEX(group_concat(URINEIODINE order by URINEIODINE), ',', floor(1+((count(URINEIODINE)-1) / 2))), ',', -1))
+
(SUBSTRING_INDEX(SUBSTRING_INDEX(group_concat(URINEIODINE order by URINEIODINE), ',', ceiling(1+((count(URINEIODINE)-1) / 2))), ',', -1)))/2
as median
FROM bs_lowiodinergn_chld_con_srvy t 

但是此方法并不是函数，使用起来有诸多限制。

SELECT ((SUBSTRING_INDEX(SUBSTRING_INDEX(group_concat(URINEIODINE order by URINEIODINE), ',', floor(1+((count(URINEIODINE)-1) / 2))), ',', -1))+(SUBSTRING_INDEX(SUBSTRING_INDEX(group_concat(URINEIODINE order by URINEIODINE), ',', ceiling(1+((count(URINEIODINE)-1) / 2))), ',', -1)))/2as medianFROM bs_lowiodinergn_chld_con_srvy t