Postgresql 按日期和特定 ID 选择前 x 行答案

【问题标题】：Posgresql SELECT first x rows by date and specific idPostgresql 按日期和特定 ID 选择前 x 行
【发布时间】：2020-08-31 04:38:07
【问题描述】：

我有这张桌子：

+----+--------+--------+-----------+----------------------------+
| id |  type  | amount | dealer_id |            date            |
+----+--------+--------+-----------+----------------------------+
| 12 | sub_s  | 29.00  |       502 | 2020-05-21 10:05:41.725+02 |
| 11 | sub_pe | 79.00  |       501 | 2020-05-20 09:05:41.725+02 |
| 10 | sub_pe | 79.00  |       501 | 2020-05-19 09:05:41.725+02 |
|  9 | sub_pe | 79.00  |       501 | 2020-05-18 09:05:41.725+02 |
|  8 | sub_pe | 79.00  |       501 | 2020-05-17 09:05:41.725+02 |
|  7 | sub_pe | 79.00  |       501 | 2020-05-16 09:05:41.725+02 |
|  6 | sub_pe | 79.00  |       501 | 2020-05-15 09:05:41.725+02 |
|  5 | sub_pe | 79.00  |       501 | 2020-05-14 09:05:41.725+02 |
|  4 | sub_pe | 79.00  |       501 | 2020-05-13 09:05:41.725+02 |
|  1 | sub_pe | 79.00  |       501 | 2020-05-12 09:05:41.725+02 |
+----+--------+--------+-----------+----------------------------+

如何只为每个dealer_id 选择前6 行（按日期ASC 排序）？

结果如下：

+----+--------+--------+-----------+----------------------------+
| id |  type  | amount | dealer_id |            date            |
+----+--------+--------+-----------+----------------------------+
| 12 | sub_s  | 29.00  |       502 | 2020-05-21 10:05:41.725+02 |
|  8 | sub_pe | 79.00  |       501 | 2020-05-17 09:05:41.725+02 |
|  7 | sub_pe | 79.00  |       501 | 2020-05-16 09:05:41.725+02 |
|  6 | sub_pe | 79.00  |       501 | 2020-05-15 09:05:41.725+02 |
|  5 | sub_pe | 79.00  |       501 | 2020-05-14 09:05:41.725+02 |
|  4 | sub_pe | 79.00  |       501 | 2020-05-13 09:05:41.725+02 |
|  1 | sub_pe | 79.00  |       501 | 2020-05-12 09:05:41.725+02 |
+----+--------+--------+-----------+----------------------------+

【问题讨论】：

标签： sql postgresql select greatest-n-per-group

【解决方案1】：

带有ROW_NUMBER()窗口功能：

select t.id, t.type, t.amount, t.dealer_id, t.date
from (
  select * ,
    row_number() over (partition by dealer_id order by date) rn
  from tablename
) t
where t.rn <= 6

【讨论】：