Create View for Latest Value Each Day (No Skipped Days) - select

Question

I have a table that records the firmware version for each device every day. If a device goes down, the script to populate the device's firmware won't be able to reach it for a value so there is no record for offline days. I need a view that will return the latest firmware version for each device every day, regardless if the device was down or not. This works great in Postgres SQL:
SELECT
d.ip,
d.date,
CASE
WHEN f.firmware_version IS NOT NULL THEN f.firmware_version
ELSE (--Use last available firmware_version for the device:
SELECT l.firmware_version
FROM firmware l
WHERE l.date < d.date AND l.firmware_version IS NOT NULL
ORDER BY l.date DESC
LIMIT 1)
END AS firmware_version
FROM
devices d --Table with a record for every device every day
LEFT JOIN firmware f ON d.date = f.date AND d.ip = f.ip
However, we are transitioning to Denodo, and I cannot get this query to work in Denodo. It seems to fail with the subquery in the case statement. Does anyone know how I can get logic like this to create a view in Denodo?

Answer 1

I figured it out! It's a bit long and complicated, but it works just the way I hoped. Here is the solution if it helps anyone else:
--Get all values of firmware prior to the listed date
--Note: Will need to find the latest firmware for each date in a later step
WITH firmware_prep (
ip,
date_main,
date_firmware,
firmware
) AS (
SELECT
d.ip,
d.date,
f.date,
f.firmware
FROM
device d LEFT JOIN
firmware f ON (d.ip = f.ip AND f.date <= d.date AND f.firmware IS NOT NULL)
)
SELECT
s.ip,
s.date_main AS date,
f.firmware
FROM
(--Here's where you find which firmware date is the latest available date for each listed date:
SELECT
ip,
date_main,
MAX(date_firmware) AS select_date
FROM
firmware_prep
GROUP BY
ip,
date_main
) s LEFT JOIN
firmware f ON s.select_date = f.date AND s.ip = f.ip