SQL-如何在列中获取唯一值（此处不起作用） [英] SQL - how to get unique values in a column (distinct does not help here)

问题描述

我有一个生产案例，我们跟踪在仓库中移动的设备。我有一张表格，显示了这些以前的位置，例如：

I have a production case where we track devices that move around warehouses. I have a table that shows these previous locations like this:

+--------+------------+-----------+------+
| device | current_WH |   date    | rank |
+--------+------------+-----------+------+
|      1 | AB         | 3/15/2018 |    7 |
|      1 | CC         | 3/19/2018 |    6 |
|      1 | CC         | 3/22/2018 |    5 |
|      1 | CC         | 3/22/2018 |    5 |
|      1 | DD         | 4/23/2018 |    4 |
|      1 | DD         | 5/11/2018 |    3 |
|      1 | DD         | 5/15/2018 |    2 |
|      1 | DD         | 5/15/2018 |    2 |
|      1 | AA         | 6/6/2018  |    1 |
|      1 | AA         | 6/6/2018  |    1 |
+--------+------------+-----------+------+

但是我需要找到current_WH的唯一值并将设备减少到一行（有数百万个设备），像这样：

But I need to find those unique values of current_WH and reduce the device to one line (there are millions of devices), like this:

+--------+------------+--------+--------+--------+
| device | current_WH | prev_1 | prev_2 | prev_3 |
+--------+------------+--------+--------+--------+
|      1 | AA         | DD     | CC     | AB     |
+--------+------------+--------+--------+--------+

我使用了等级函数（按日期按设备顺序分组）。它几乎做到了，但不完全是。您无法对current_WH进行排名，因为它将按字母顺序排名。我需要按时间对current_WH进行排名。

I used the rank function (group by device order by date). It almost does it but not quite. You can't rank current_WH because that will rank alphabetically. I need to rank current_WH by time.

有人知道如何实现第二张桌子吗？谢谢你的帮助！

Any idea how to achieve the second table? Thanks for your help!

推荐答案

我认为这符合您的要求：

I think this does what you want:

select device,
       max(case when seqnum = 1 then current_wh end) as current_wh,
       max(case when seqnum = 2 then current_wh end) as prev_1_wh,
       max(case when seqnum = 3 then current_wh end) as prev_2_wh,
       max(case when seqnum = 4 then current_wh end) as prev_3_wh
from (select t.*, dense_rank() over (partition by device order by maxdate desc) as seqnum
      from (select t.*, max(date) over (partition by device, current_wh) as maxdate
            from t
           ) t
     ) t
group by device

这篇关于SQL-如何在列中获取唯一值（此处不起作用）的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！