使用窗口函数 LAG() 时如何保留第一条记录？答案

【问题标题】：How to retain the first record while using Window function LAG()?使用窗口函数 LAG() 时如何保留第一条记录？
【发布时间】：2021-11-11 22:27:50
【问题描述】：

我的数据看起来像 -

实际表 -

VIN	Mode	Status	Start	End
ABC123456789	Mode 1	Waiting for Auth	01/01/2010 00:00:00	05/05/2014 14:54:54
ABC123456789	Mode 1	Waiting for URL	05/05/2014 14:54:54	05/13/2014 19:09:51
ABC123456789	Mode 1	Waiting for User	05/13/2014 19:09:51	11/13/2014 22:26:32
ABC123456789	Mode 1	Authorized	11/13/2014 22:26:32	11/13/2014 22:31:00
ABC123456789	Mode 1	Authorized	11/13/2014 22:31:00	11/14/2014 01:23:56
ABC123456789	Mode 2	Waiting for User	11/14/2014 01:23:56	11/18/2014 19:38:51
ABC123456789	Mode 2	Waiting for User	11/18/2014 19:38:51	11/18/2014 19:38:54
ABC123456789	Mode 2	Waiting for User	11/18/2014 19:38:54	11/18/2014 20:07:52
ABC123456789	Mode 2	Authorized All	11/18/2014 20:07:52	12/17/2014 19:22:50
ABC123456789	Mode 2	Authorized All	12/17/2014 19:22:50	02/25/2015 20:03:44
ABC123456789	Mode 2	Authorized All	02/25/2015 20:03:44	02/25/2015 20:03:48
ABC123456789	Mode 3	Authorized All	02/25/2015 20:03:48	02/25/2015 20:14:05
ABC123456789	Mode 3	Revoke Auth	02/25/2015 20:14:05	02/25/2015 20:14:29
ABC123456789	Mode 3	Waiting for Auth	02/25/2015 20:14:29	02/25/2015 20:40:21

我正在使用下面的窗口函数查询来获得预期的结果，如结果输出表所示。 但我无法保留第一行。如何实现？

Hive 查询 -

WITH mma AS
(
select VIN, 
       Mode, 
       Status,
       case when lower(Status) like '%authorized%' then 'Authorized' else 'Deauthorized' end Event,
       Start, 
       End 
  from ModemAuth 
 where VIN = 'ABC123456789'
 order by Start
 )

select mma2.*
from (select mma.*,
             lag(event) over (partition by VIN order by Start) as Prev_Event
        from mma
     ) mma2
where Prev_Event <> Event

预期结果 -

VIN	Mode	Status	Event	Start	End
ABC123456789	Mode 1	Waiting for Auth	Deauthorized	01/01/2010 00:00:00	05/05/2014 14:54:54
ABC123456789	Mode 1	Authorized	Authorized	11/13/2014 22:26:32	11/13/2014 22:31:00
ABC123456789	Mode 2	Waiting for User	Deauthorized	11/14/2014 01:23:56	11/18/2014 19:38:51
ABC123456789	Mode 2	Authorized All	Authorized	11/18/2014 20:07:52	12/17/2014 19:22:50
ABC123456789	Mode 3	Revoke Auth	Deauthorized	02/25/2015 20:14:05	02/25/2015 20:14:29

【问题讨论】：

保留第一行是什么意思？你得到了什么？

标签： sql hive common-table-expression

【解决方案1】：

您的where 条件不正确。应该是：

where Prev_Event is null or prev_event <> Event

这是我对你之前的逻辑，类似question。你可以接受这个答案。

【讨论】：

谢谢戈登！我完全错过了您之前解决方案中的 NULL 条件。这很有帮助。我接受这两种解决方案。