mysql-求中位數方法

mysql中並沒有直接求中位數的函數，經過一番查找之后，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 

但是此方法並不是函數，使用起來有諸多限制。

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