Re: Window function sort order help

Dianna Harter <dharter(at)mynewplace(dot)com> wrote:

> Having trouble getting a window function to sort correctly.

> Given this data

> consumer_id | move_date | history_timestamp
> -------------+------------+----------------------------
> 12345| 2008-01-05 | 2007-12-11 06:02:26.842171
> 12345| 2008-02-29 | 2008-02-05 07:22:38.04067
> 12345| 2008-02-29 | 2008-07-11 09:03:42.44044
> 23456| 2009-01-01 | 2008-11-12 07:33:32.656658 <--
> 23456| 2009-01-28 | 2008-11-14 01:57:40.264335
> 23456| 2009-01-01 | 2008-12-04 17:14:20.279999 <--
> 23456| 2009-01-01 | 2008-12-31 00:33:37.204968
> 23456| 2009-01-01 | 2011-06-08 04:16:41.646521
> 34567| 2010-05-07 | 2010-06-08 05:14:43.842172

> I'm trying to get the timestamp when the consumer last changed their move_date. (Notice consumer_id 23456 set their move_date to 2009-01-01 then changed and then changed it back. In the end, I want the timestamp from when they changed it to 2009-01-01 the second time.)

> My thought was to do an intermediary step to find the timestamp for each time it switched. From there I can grab the max(timestamp) for each consumer.

> [...]

> Any suggestions to get the order by to occur first then the partition by or maybe there another approach that I could use?

If I understand the question correctly, try:

| SELECT DISTINCT ON (consumer_id) consumer_id, move_date, history_timestamp
| FROM (SELECT consumer_id, move_date, history_timestamp,
| LAG(move_date) OVER (PARTITION BY consumer_id ORDER BY consumer_id, history_timestamp) AS previous_move_date
| FROM consumer_hist) AS SubQuery
| WHERE move_date IS DISTINCT FROM previous_move_date
| ORDER BY consumer_id, history_timestamp DESC;

Tim