如何从开始日期和结束日期识别和聚合序列答案

【问题标题】：How to identify and aggregate sequence from start and end dates如何从开始日期和结束日期识别和聚合序列
【发布时间】：2017-01-06 14:26:58
【问题描述】：

我正在尝试根据person 识别日期中的连续序列，以及该序列的总和amount。我的records 表如下所示：

person   start_date   end_date     amount
1        2015-09-10   2015-09-11   500
1        2015-09-11   2015-09-12   100
1        2015-09-13   2015-09-14   200
1        2015-10-05   2015-10-07   2000
2        2015-10-05   2015-10-05   300
2        2015-10-06   2015-10-06   1000
3        2015-04-23   2015-04-23   900

结果查询应该是这样的：

person   sequence_start_date   sequence_end_date     amount
1        2015-09-10            2015-09-14            800
1        2015-10-05            2015-10-07            2000
2        2015-10-05            2015-10-06            1400
3        2015-04-23            2015-04-23            900

下面，我可以使用 LAG 和 LEAD 来识别序列 start_date 和 end_date，但我没有办法聚合 amount。我假设答案将涉及某种ROW_NUMBER() 窗口函数，该函数将按序列分区，我只是不知道如何使序列可被函数识别。

SELECT
 person
 ,COALESCE(sequence_start_date, LAG(sequence_start_date, 1) OVER (ORDER BY person, start_date)) AS "sequence_start_date"
 ,COALESCE(sequence_end_date, LEAD(sequence_end_date, 1) OVER (ORDER BY person, start_date)) AS "sequence_end_date"
FROM
(
 SELECT
  person
  ,start_date
  ,end_date
  ,CASE WHEN LAG(end_date, 1) OVER (PARTITION BY person ORDER BY start_date) + interval '1 day' = start_date
   THEN NULL
   ELSE start_date
  END AS "sequence_start_date"
  ,CASE WHEN LEAD(start_date, 1) OVER (PARTITION BY person ORDER BY start_date) - interval '1 day' = end_date
   THEN NULL
   ELSE end_date
  END AS "sequence_end_date"
  ,amount
 FROM records
) sq

【问题讨论】：

现有查询中的子查询生成的sequence_start_date 和sequence_end_date 值与这些列名不一致。
您的数据似乎有点不一致。在某些情况下，一行的开始日期与前一行的结束日期相同，顺序相同，但在其他情况下，开始日期比前一个结束日期晚一天。这是数据中的错误，还是两种情况都需要考虑？

标签： sql postgresql window-functions gaps-and-islands

【解决方案1】：

即使您更新的（子）查询仍然不完全适合您提供的数据，这对于序列中第二行和后续行的开始日期是否应等于其前一行的结束日期不一致日期或一天后。如果需要，可以很容易地更新查询以适应两者。

在任何情况下，您都不能将 COALESCE 用作窗口函数。通过提供OVER 子句，聚合函数可以用作窗口函数，但不是普通函数。尽管如此，还是有一些方法可以将窗口函数应用于此任务。这是一种识别数据中序列的方法（如图所示）：

SELECT
  person
  ,MAX(sequence_start_date)
     OVER (
       PARTITION BY person
       ORDER BY start_date
       ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW)
     AS "sequence_start_date"
  ,MIN(sequence_end_date)
     OVER (
       PARTITION BY person
       ORDER BY start_date
       ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING)
     AS "sequence_end_date"
  ,amount
FROM
(
 SELECT
  person
  ,start_date
  ,end_date
  ,CASE WHEN LAG(end_date, 1) OVER (PARTITION BY person ORDER BY start_date) + interval '1 day' >= start_date
   THEN date '0001-01-01'
   ELSE start_date
   END AS "sequence_start_date"
  ,CASE WHEN LEAD(start_date, 1) OVER (PARTITION BY person ORDER BY start_date) - interval '1 day' <= end_date
   THEN NULL
   ELSE end_date
   END AS "sequence_end_date"
  ,amount
 FROM records
 order by person, start_date
) sq_part
ORDER BY person, sequence_start_date

这依赖于MAX() 和MIN() 而不是COALESCE()，它应用窗口框架来为每个分区中的每个对象获取适当的范围。结果：

person  sequence_start_date         sequence_end_date           amount
1       September, 10 2015 00:00:00 September, 12 2015 00:00:00 500
1       September, 10 2015 00:00:00 September, 12 2015 00:00:00 100
1       October, 05 2015 00:00:00   October, 07 2015 00:00:00   2000
2       October, 05 2015 00:00:00   October, 06 2015 00:00:00   300
2       October, 05 2015 00:00:00   October, 06 2015 00:00:00   1000
3       April, 23 2015 00:00:00     April, 23 2015 00:00:00     900

请注意，这不需要结束日期与后续开始日期完全匹配；每个人相邻或重叠的所有行都将被分配到相同的序列。但是，如果 (person, start_date) 不能被认为是唯一的，那么您可能还需要按结束日期对分区进行排序。

现在您有了一种识别序列的方法：它们的特征是三元组person, sequence_start_date, sequence_end_date。（或者实际上，您只需要这些日期中的个人和一个来进行识别，但请继续阅读。）您可以将上述查询包装为外部聚合查询的内联视图，以产生您想要的结果：

SELECT
  person,
  sequence_start_date,
  sequence_end_date,
  SUM(amount) AS "amount"
FROM ( <above query> ) sq
GROUP BY person, sequence_start_date, sequence_end_date

当然，如果要选择两个日期，则需要将它们作为分组列。

【讨论】：

【解决方案2】：

为什么不：

select a1.person, a1.sequence_start_date, a1.sequence_end_date, 
       sum(rx.amount) 
         as amount
from (EXISTING_QUERY) a1
left join records rx 
  on rx.person = a1.person 
  and rx.start_date >= a1.start_date
  and rx.end_date <= a1.end_date
group by a1.person, a1.sequence_start_date, a1.sequence_end_date

【讨论】：