SQL连接查询中位数

Question

我创建了一个包含两个连接表的查询，我希望从一列中返回中值。

查询看起来像这样：

select table1.column1, count(distinct(table2.column2)) 
 from table1 left join 
 table2 on table1.column1 = table2.column4
 where column3 = 1 
 group by table1.column1

结果看起来像这样（有更多行的c）：

|   column1  | column2 |
 +------------+---------+
 |    111     |    4    | 
 |    222     |    5    |
 |    333     |    5    |
 |    444     |    5    |

我想从第2列结果中提取中位数。

有没有办法在没有对此查询进行重大修改的情况下执行此操作？

Answer 1

你可以使用percentile_disc()：

select percentile_disc(0.5) over (order by cnt)
from (select table1.column1, count(distinct table2.column2) as cnt
      from table1 left join
           table2
           on table1.column1 = table2.column4
      where column3 = 1
      group by table1.column1
     ) t

Answer 2

请创建以下函数以获得中位数：

CREATE OR REPLACE FUNCTION _final_median(NUMERIC[])
   RETURNS NUMERIC AS
$$
   SELECT AVG(val)
   FROM (
     SELECT val
     FROM unnest($1) val
     ORDER BY 1
     LIMIT  2 - MOD(array_upper($1, 1), 2)
     OFFSET CEIL(array_upper($1, 1) / 2.0) - 1
   ) sub;
$$
LANGUAGE 'sql' IMMUTABLE;

CREATE AGGREGATE median(NUMERIC) (
  SFUNC=array_append,
  STYPE=NUMERIC[],
  FINALFUNC=_final_median,
  INITCOND='{}'
);

用法示例：SELECT median(num_value) AS median_value FROM t;

基于以下问题，具体针对您：

select t.*,median(column2) as median_value
from (
      select table1.column1, count(distinct(table2.column2)) as column2
      from table1 left join 
      table2 on table1.column1 = table2.column4
      where column3 = 1 
      group by table1.column1
     ) t

参考：https://wiki.postgresql.org/wiki/Aggregate_Median

更多例子：How do i get min, median and max from my query in postgresql