中位数函数：MEDIAN
使用表达式：MEDIAN ( expression ) OVER ( [ window‑partition‑clause ] )
准备测试数据：
CREATE TABLE allsales(state VARCHAR(20), name VARCHAR(20), sales INT);
INSERT INTO allsales VALUES('MA', 'A', 60);
INSERT INTO allsales VALUES('NY', 'B', 20);
INSERT INTO allsales VALUES('NY', 'C', 15);
INSERT INTO allsales VALUES('MA', 'D', 20);
INSERT INTO allsales VALUES('MA', 'E', 50);
INSERT INTO allsales VALUES('NY', 'F', 40);
INSERT INTO allsales VALUES('MA', 'G', 10);
COMMIT;
最简单的用法：
=> SELECT name, sales, MEDIAN(sales) OVER() AS median FROM allsales;
name | sales | median
------+-------+--------
G | 10 | 20
C | 15 | 20
D | 20 | 20
B | 20 | 20
F | 40 | 20
E | 50 | 20
A | 60 | 20
可以按照指定字段分组，显示所在分组的中位数，如下：
=> SELECT state, name, sales, MEDIAN(sales) OVER(partition by state) AS median FROM allsales;
state | name | sales | median
-------+------+-------+--------
MA | G | 10 | 35
MA | D | 20 | 35
MA | E | 50 | 35
MA | A | 60 | 35
NY | C | 15 | 20
NY | B | 20 | 20
NY | F | 40 | 20
官方文档：