在Postgres中求数列的长度 [英] Finding the length of a series in postgres
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问题描述
Postgres的棘手查询。想象一下,我有一组行,其中包含一个名为(例如)Success的布尔列。如下所示:
id | success 9 | false 8 | false 7 | true 6 | true 5 | true 4 | false 3 | false 2 | true 1 | false
我需要计算最新的(不成功的)系列的长度。例如,在这种情况下,它将是"3"表示成功,而"2"表示不成功。或使用窗口函数,然后类似于:
id | success | length 9 | false | 2 8 | false | 2 7 | true | 3 6 | true | 3 5 | true | 3 4 | false | 1 3 | true | 2 2 | true | 2 1 | false | 1
(请注意,我通常只需要最新系列的长度,而不是所有系列)
到目前为止,我找到的最接近的答案是这篇文章: https://jaxenter.com/10-sql-tricks-that-you-didnt-think-were-possible-125934.html (见#5) 但是,Postgres不支持"Ignore Null"选项,因此查询无法工作。如果没有"忽略空值",它只会在"长度"列中返回空值。这是我能得到的最接近的:
WITH
trx1(id, success, rn) AS (
SELECT id, success, row_number() OVER (ORDER BY id desc)
FROM results
),
trx2(id, success, rn, lo, hi) AS (
SELECT trx1.*,
CASE WHEN coalesce(lag(success) OVER (ORDER BY id DESC), FALSE) != success THEN rn END,
CASE WHEN coalesce(lead(success) OVER (ORDER BY id DESC), FALSE) != success THEN rn END
FROM trx1
)
SELECT trx2.*, 1
- last_value (lo) IGNORE nulls OVER (ORDER BY id DESC ROWS BETWEEN
UNBOUNDED PRECEDING AND CURRENT ROW)
+ first_value(hi) OVER (ORDER BY id DESC ROWS BETWEEN CURRENT ROW
AND UNBOUNDED FOLLOWING)
AS length FROM trx2;
您对这样的查询有什么想法吗?
推荐答案
可以使用窗口函数row_number()
指定系列:
select max(id) as max_id, success, count(*) as length
from (
select *, row_number() over wa - row_number() over wp as grp
from my_table
window
wp as (partition by success order by id desc),
wa as (order by id desc)
) s
group by success, grp
order by 1 desc
max_id | success | length
--------+---------+--------
9 | f | 2
7 | t | 3
4 | f | 2
2 | t | 1
1 | f | 1
(5 rows)
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