SQL Retrieve latest value relative to a subset - sql

Question

I'm trying to run a SQL query that pulls in latest result relative to a subset.
GIVEN Device 1 is linked to Device 3, I would like the query to show Device 1's color as Device 3's because Device 1 is linked to Device 3 and Device 3's color has been changed most recently.
Current results:
Device | Color
-------+---------
1 | red
2 | blue
3 | green
What I would like:
Device | Color
-------+---------
1 | GREEN
2 | blue
3 | GREEN
DeviceTable
account_id
----------
1
2
3
BeaconsTable
account_id | Color | Time
-----------+--------+--------
1 | red | 6:00
2 | blue | 7:00
3 | red | 8:00
3 | green | 10:00
LinkTable
account_id | Link
------------+--------
1 | 3
2 | -
3 | -
Here's what I am doing right now and again, this results in the latest color for the device, but not for the linked device.
SELECT d.* , b.*
FROM Device d
JOIN Beacons b ON d.account_id = b.account_id
JOIN (
SELECT b.account_id,
MAX(b.timestamp) as max_date
FROM Beacons b
GROUP BY b.account_id) x ON x.account_id = d.account_id
AND x.max_date = b.timestamp

Answer 1

If I am following this logic correctly, you want to join to the links and use the link there, under some circumstances:
SELECT COALESCE(l.link, d.account_id) as device, b.color
FROM Device d LEFT JOIN
Links l
ON d.account_id = l.account_id LEFT JOIN
(SELECT b.*,
ROW_NUMBER() OVER (PARTITION BY b.account_id ORDER BY b.time DESC) as seqnum
FROM beacons b
) b
ON b.account_id = COALESCE(l.link, d.account_id);

Answer 2

Join the tables and then LEFT join 1 more copy of Beacons to get the potential color of the linked device.
Then use window function FIRST_VALUE() to get that color:
select distinct d.account_id,
first_value(coalesce(b2.Color, b.Color)) over (partition by d.account_id order by coalesce(b2.Time, b.Time) desc) Color
from Device d
inner join Beacons b on b.account_id = d.account_id
inner join Link l on l.account_id = d.account_id
left join Beacons b2 on b2.account_id = l.Link and b2.Time > b.Time
note that the column Time must be in the format hh:mm so it is comparable.
See the demo.
Results:
> account_id | Color
> ---------: | :----
> 1 | green
> 2 | blue
> 3 | green

Answer 3

This should work:
SELECT
account_id,
color
FROM Device AS d
JOIN (
SELECT
linked_id AS account_id,
color,
ROW_NUMBER() OVER (PARTITION BY linked_id ORDER BY Time DESC) AS row_number
FROM Beacons AS b
JOIN (
SELECT
account_id AS linked_id,
Link AS account_id
FROM Linked
) USING (account_id)
) USING (account_id)
WHERE row_number = 1
;
Here's a Demo