如何使用SQL窗口函数计算聚合的百分比

Question

我需要计算表中各种尺寸的百分比.我想通过使用窗口函数来计算分母来简化事情,但是我遇到了问题,因为分子也必须是聚合.

举个简单的例子,请看下表:

create temp table test (d1 text, d2 text, v numeric);
insert into test values ('a','x',5), ('a','y',5), ('a','y',10), ('b','x',20);

如果我只想计算d1中每一行的份额,那么窗口函数可以正常工作:

select d1, d2, v/sum(v) over (partition by d1)
from test;

"b";"x";1.00
"a";"x";0.25
"a";"y";0.25
"a";"y";0.50

但是,我需要做的是计算d1中d2之和的总份额.我正在寻找的输出是这样的:

"b";"x";1.00
"a";"x";0.25
"a";"y";0.75

所以我试试这个:

select d1, d2, sum(v)/sum(v) over (partition by d1)
from test
group by d1, d2;

但是,现在我收到一个错误:

ERROR:  column "test.v" must appear in the GROUP BY clause or be used in an aggregate function

我假设这是因为它抱怨在分组子句中没有考虑窗口函数,但是无论如何窗口函数都不能放在分组子句中.

这是使用Greenplum 4.1,它是Postgresql 8.4的一个分支,并共享相同的窗口函数.请注意,Greenplum无法执行相关子查询.

Answer 1

我认为你真正想要的是:

SELECT d1, d2, sum(v)/sum(sum(v)) OVER (PARTITION BY d1) AS share
FROM   test
GROUP  BY d1, d2;

产生请求的结果.

窗口函数在聚合函数之后应用.在这个例子中,外部sum()in sum(sum(v))是一个窗口函数,它附加到OVER ...子句,而内部sum()是一个聚合.

实际上与:

WITH x AS (
    SELECT d1, d2, sum(v) AS sv
    FROM   test
    GROUP  BY d1, d2
    )
SELECT d1, d2, sv/sum(sv) OVER (PARTITION BY d1) AS share
FROM   x;

或者(没有CTE):

SELECT d1, d2, sv/sum(sv) OVER (PARTITION BY d1) AS share
FROM   (
    SELECT d1, d2, sum(v) AS sv
    FROM   test
    GROUP  BY d1, d2
    ) x;

或@ Mu的变种.

旁白:Greenplum引入了4.2版的相关子查询.请参阅发行说明.