SQL查询根据给定多条记录的不同最大值获取最大值 [英] SQL Query to Get Max Value Based on Different Max Value Given Multiple Records

问题描述

我知道我可以用 CTE 或其他方式来做到这一点，但我想知道这是否可以在一个选择查询中完成(没有子选择).我想为 po_nbr 找到最新的 crt_ts，然后获取关联的 id.

I know I can do this with a CTE or another means but I am wondering if this can be done in one select query (no sub-selects). I want to find the most recent crt_ts for the po_nbr and then take the associated id.

id 列不能保证是连续的，我为这个问题简化了它.

The id column is not guaranteed to be sequential, I simplified it for this question.

create table temp
(
    id int,
    po_nbr int,
    crt_ts datetime
)

insert into temp values (20, 100, '09/01/2009')
insert into temp values (3, 100, '09/03/2009')
insert into temp values (5, 100, '09/05/2009')
insert into temp values (6, 100, '09/07/2009')
insert into temp values (4, 200, '08/01/2009')
insert into temp values (29, 200, '08/03/2009')
insert into temp values (12, 200, '08/05/2009')
insert into temp values (18, 200, '08/07/2009')

---

预期结果:

id  po_nbr
---------
6   100
18  200

推荐答案

SELECT
  MAX(id) id,
  po_nbr
FROM
  temp
GROUP BY
  po_nbr

要获得关联的日期，您可以这样做(请注意，这意味着顺序 ID):

To have the associated date, you could do (beware, this implies a sequential id):

SELECT
  temp.id,
  temp.po_nbr,
  temp.crt_ts
FROM
  temp
  INNER JOIN (
    SELECT MAX(id) id FROM temp GROUP BY po_nbr
  ) latest ON latest.id = temp.id

如果没有连续的 id，那就是:

Without a sequential id, it would be:

SELECT
  MAX(temp.id) id,
  temp.po_nbr,
  temp.crt_ts
FROM
  temp INNER JOIN (
    SELECT   MAX(crt_ts) crt_ts, po_nbr 
    FROM     temp i
    GROUP BY po_nbr
  ) latest ON latest.crt_ts = temp.crt_ts AND latest.po_nbr = temp.po_nbr
GROUP BY
  temp.po_nbr,
  temp.crt_ts

如果保证每个 po_nbr 组没有两个相等的日期，则可以省略 GROUP BY.

The GROUP BY can be left out if there are guaranteed to be no two equal dates per po_nbr group.

crt_ts 和 po_nbr 上的索引有助于最后一个查询，最好创建一个组合索引.

Indexes on crt_ts and po_nbr help in the last query, creating one combined index would be best.

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