查找当前行是否是要从数据库中选择的最后一行 [英] Finding if current row is last row to be selected from database

问题描述

我正在从数据库中选择期间列表。如果当前行是第一行，则周期以日期开头，我可以找到周期开始之间的间隔，如下所示：

I am selecting list of periods from database. If current row is first row then the period starts with date and I can find the interval between period start like this:

SELECT
...
CASE WHEN row_number() OVER(ORDER BY r.created_at ASC) = 1 THEN r.created_at - r.created_at::date ELSE NULL END AS period
...
FROM mytable r

我如何对 last 行？以查找最后一行的r.created_at与日期的午夜之间的时间。

How can I do the same to last row? To find the time between the r.created_at of last row and midnight of its date.

我知道<$ c $ PostgreSQL中的c> first 和 last 函数（ https://wiki.postgresql.org/wiki/First/last_（合计）），但它们是合计函数，在这种情况下无济于事。

I am aware of first and last functions in PostgreSQL (https://wiki.postgresql.org/wiki/First/last_(aggregate)), but they are aggregate functions and do not help in this case.

编辑：
这个问题有2个很好的答案。在我的情况下，它们都不起作用，因为我作为问题的一部分提出的这一行是较大查询的一部分，以编程方式组合在一起，使用提供的解决方案将迫使我更改很多代码，而我不愿意这样做。这点。如果遇到扩展问题，那么我肯定会重新考虑。

This question has 2 great answers. Neither of them help in my case, as this single line i presented as part of my question is part of bigger query, put together programmatically and using the solutions offered would force me to alter alot of code, which i am not willing to do at this point. Should the scaling problems hit - then i will certainly reconsider.

推荐答案

这可能比窗口函数要快：

This might be faster than window functions:

with r as (
    select
        min(created_at) as min_created_at,
        max(created_at) as max_created_at
    from mytable
)
select
    case when (select min_created_at from r) = created_at
    then created_at - created_at::date else null
    end as period_min,
    case when (select max_created_at from r) = created_at
    then created_at - created_at::date else null
    end as period_max
from mytable

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