Join two tables. Use a joined column as Group By to then select the latest row based on timestamp

Question

I have two tables:

connector_status

connector	status_timestamp	status
1	2020-03-03 09:07:09.058000	available
2	2020-03-03 09:51:03.852000	faulted
1	2022-10-06 16:32:14.130000	charging
3	2022-10-06 16:28:26.228000	available
4	2022-10-06 16:28:03.195000	charging

connector

connector	box_id	connector_id
1	Alpha	0
2	Alpha	1
3	Beta	0
4	Beta	1

My connector_status table has multiple rows for each connector, but I only want the most recent row, based on the box_id

I would like to join the tables based on box_id but using the latest timestamp from the 2x connectors. This would select status charging based on the above table

The joined table would look a bit like this:

connector	status_timestamp	status	box_id
1	2020-03-03 09:07:09.058000	available	Alpha
2	2020-03-03 09:51:03.852000	faulted	Alpha
1	2022-10-06 16:32:14.130000	charging	Alpha
3	2022-10-06 16:28:26.228000	available	Beta
4	2022-10-06 16:28:03.195000	charging	Beta

With the desired result:

box_id	status
Alpha	Charging
Beta	Available

I have the following code

SELECT IF(connector_status.status = 'Charging','Charging', IF(connector_status.status ='Available','Not Occupied', IF(connector_status.status = 'Faulted','Faulted','Occupied'))) AS group_status, connector.connector_id, connector.box_id, status_timestamp FROM connector_status JOIN connector ON connector_status.connector = connector.connector GROUP BY connector.box_id ORDER BY connector.box_id

I don't know how to do the join on box_id to get the max timestamp though.

What is confusing me is if i firstly get the latest timestamp from the connector_status table and then try and join by box_id, how can i be sure that it will take the latest connector timestanmp for that box_id

Answer 1

You could use row_number to get the desired result:

select box_id,
       status       
from (   select status,
                box_id,
                row_number() over( partition by tbl.box_id order by tbl.status_timestamp desc ) as rn
         from   (  select   status_timestamp,
                            status,
                            box_id
                   from connector_status cs
                   inner join connector c on c.connector=cs.connector 
                ) as tbl
     ) as x where rn=1 ;

https://dbfiddle.uk/1LRcbbma