Resolved: Retrieve SQL records where only the last unique entries match criteria in postgresql

Question:

I’ve got a long table that tracks a numerical ‘state’ value (0=new, 1=setup mode, 2=retired, 3=active, 4=inactive) of a collection of ‘devices’ historically. These devices may be activated/deactivated throughout the year, so the table is continuous collection of state changes – mostly state 3 and 4, ordered by id, with a timestamp on the end, for example:

     id    | device_id | new_state |        when         
 ----------+-----------+-----------+----------------------------
 218010581 |      2505 |         0 | 2022-06-06 16:28:11.174084
 218010580 |      2505 |         1 | 2022-06-06 16:28:11.174084
 218010634 |      2505 |         3 | 2022-06-06 16:29:25.129019
 218087737 |       659 |         3 | 2022-06-07 22:55:48.705208
 218087744 |      1392 |         3 | 2022-06-07 22:55:59.016974
 218087757 |      1556 |         3 | 2022-06-07 22:56:09.811876
 218087758 |      2071 |         1 | 2022-06-07 22:56:20.850095
 218087765 |      2071 |         3 | 2022-06-07 22:56:29.122074

When I want to look for a list of devices and see their ‘history’, I know I can use something like:

select * 
from devstatechange 
where device_id = 2345 
order by "when";

    id     | device_id | new_state |            when            
-----------+-----------+-----------+----------------------------
 184682659 |      2345 |         0 | 2021-05-27 17:03:36.894429
 184682658 |      2345 |         1 | 2021-05-27 17:03:36.894429
 184684721 |      2345 |         3 | 2021-05-27 17:31:01.968314
 194933399 |      2345 |         4 | 2021-08-31 23:30:05.555407
 195213746 |      2345 |         3 | 2021-09-03 16:53:39.043005
 206278232 |      2345 |         4 | 2021-12-31 22:30:08.820068
 206515355 |      2345 |         3 | 2022-01-03 16:06:01.223759
 215709888 |      2345 |         4 | 2022-04-30 23:30:30.309389
 215846807 |      2345 |         3 | 2022-05-02 19:40:31.525514


select * 
from devstatechange 
where device_id = 2351 
order by "when";

    id     | device_id | new_state |            when            
-----------+-----------+-----------+----------------------------
 186091252 |      2351 |         0 | 2021-06-09 15:36:02.775035
 186091253 |      2351 |         1 | 2021-06-09 15:36:02.775035
 186091349 |      2351 |         3 | 2021-06-09 15:37:56.965599
 197880878 |      2351 |         4 | 2021-09-30 23:30:06.691835
 197945073 |      2351 |         3 | 2021-10-01 15:32:35.907913
 208981857 |      2351 |         4 | 2022-01-31 22:30:09.521694
 209722639 |      2351 |         3 | 2022-02-09 15:20:12.412816
 217666572 |      2351 |         4 | 2022-05-31 23:30:30.881928

What I am really looking for is a query that returns a unique list of devices where the latest dated entry for each device only contains a state of ‘4’ (‘inactive state’), and not include records that do not match.
So in using the above data samples, even though both devices 2345 and 2351 have states of 3 and 4 throughout their history, only device 2351 has it’s last dated entry with a state of 4 – meaning it is currently in an ‘inactive’ state. Device 2345’s would not appear in the result set since its last dated entry has a state of 3 – it’s still active.
Stabbing in the dark, I’ve tried variants of:

SELECT DISTINCT * 
FROM devstatechange 
WHERE MAX("when") AND new_state = 4 
ORDER BY "when";

SELECT DISTINCT device_id, new_state, MAX("when") 
FROM devstatechange 
WHERE new_state = 4  
ORDER BY "when";

with obviously no success.
I’m thinking I might need to ‘group’ the entries together, but I don’t know how to specify ‘return last entry only if new_state = 4′ in SQL, or rather PostgreSQL.
Any tidbits or pokes in the right direction would be appreciated.

Answer:

SELECT * FROM (
  SELECT DISTINCT ON (device_id)
  *
  FROM devstatechange
  ORDER BY device_id, "when" DESC
)
WHERE new_state = 4;

The DISTINCT ON keyword together with the ORDER BY will pull the newest row for each device. The outer query then filters these by your condition.

If you have better answer, please add a comment about this, thank you!

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