Postgresql 选择最后一组的第一项

Question

我有这个数据：

ID | Amount
1 | 0
2 | 0
3 | 0
4 | 0 ---> NULL

ID | Amount
1 | -1 ---> this row
2 | 0
3 | 0
4 | 0
5 | 0

ID | Amount
1 | 0
2 | 0
3 | -1 ---> this row
4 | 0
5 | 0
6 | 0
7 | 0

ID | Amount
1 | 0
2 | -1
3 | -1
4 | 0
5 | 0
6 | -1 ---> this row
7 | 0
8 | 0

ID | Amount
1 | 0
2 | -1
3 | 0
4 | 0
5 | 0
6 | -1 ---> this row
7 | -1
8 | 0

我的逻辑是从最后开始，向上移动直到我们达到第一个 -1。但是，我无法为此得出 SQL。感谢您的帮助！

Answer 1

假设您有 -1 个，您可以通过计算后面的 0 个来对它们进行分组。然后可以使用 dense_rank():

枚举这些

select id, rnk
from (select t.*, dense_rank() over (partition by grp) as rnk
      from (select t.*,
                   count(*) filter (where amount = 0) over (order by id desc) as grp
            from t
           ) t
     ) t
where amount = -1 and rnk = 1
order by id
fetch first 1 row only;

要处理您的第一个案例，您可以在这样的查询中合并：

select id, rnk
from t
where not exists (select 1 from t where amount = -1)
order by id
fetch first 1 row only

Postgresql 选择最后一组的第一项

Postgresql selecting first item of last group

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