---

前言

从 STATION 查询北纬度 （LAT_N） 的中位数，并将您的答案四舍五入到小数点后4位.
中位数的定义是：如果数据量是奇数，则中位数是排序后位于中间的数；如果数据量是偶数，则中位数是排序后中间两个数的平均值。
STATION表说明如下：

---

一、题析

以MySQL为例：
给每一行加一列行号

@rownum:=@rownum+1 as `row_number`

判断奇偶数的情况

1、获取总数：

SELECT count(*) as total_rows
  FROM station d

如果是奇数：(总行数+1)/2；偶数：总行数/2 和 总行数/2 +1
进行合并：

floor((total_rows+1)/2), floor((total_rows+2)/2) )

四舍五入到四位小数：

truncate(AVG(LAT_N),4)

二、题解

1.mysql

代码如下：

select truncate((SELECT avg(t1.LAT_N) as median_val FROM (
SELECT @rownum:=@rownum+1 as `row_number`, d.LAT_N
  FROM station d,  (SELECT @rownum:=0) r
  ORDER BY d.LAT_N
) as t1, 
(
  SELECT count(*) as total_rows
  FROM station d
) as t2
WHERE  t1.row_number in ( floor((total_rows+1)/2), floor((total_rows+2)/2) )),4);

2.sqlserver

SELECT CAST( AVG(LAT_N) AS NUMERIC(20,4))
FROM
(
   SELECT LAT_N, 
      ra = ROW_NUMBER() OVER (ORDER BY LAT_N, id),
      rd = ROW_NUMBER() OVER (ORDER BY LAT_N DESC, id DESC)
   FROM STATION
) AS x
WHERE ra BETWEEN rd - 1 AND rd + 1