How to filter results - mysql

Question

I have a query which is used to obtain information about the owner of a vehicle at a particular point in time, when it is sighted (from the vehicle_sightings table). I have attached a snippet of part of the query below:
SELECT
sighting_id
FROM
vehicle_sightings
INNER JOIN
vehicle_vrn ON vehicle_sightings.plate = vehicle_vrn.vrnno
INNER JOIN
vehicle_ownership ON vehicle_vrn.fk_sysno = vehicle_ownership.fk_sysno
WHERE
vehicle_sightings.seenDate >= vehicle_ownership.ownership_start_date
AND (vehicle_sightings.seenDate <= vehicle_ownership.ownership_end_date
OR vehicle_ownership.ownership_end_date IS NULL
OR vehicle_ownership.ownership_end_date = '0001-01-01 00:00:00')
This works well for most scenarios where a vehicle only has one owner in its history. However, there are some instances where the ownership_end_date field is not filled in (it is filled in for most cases, as it indicates that a vehicle would have changed hands, and is from that stage onwards passed on to a new owner). In these instances where it is not filled in (or left default), all entries for that ownership history are returned, such as the below case:
In that case above, the query returns both of those records as the seenDate fits in both of them since the end date is not filled in (and has the default value in that case). I therefore need to modify my query to return the record with the highest ownership_start_date in those cases.
I tried to do this by adding the following at the end:
GROUP BY sighting_id HAVING seenDate >= MAX(ownership_start_date)
This however did not work, as many less records were returned. Is there a clean way this can be achieved, maybe without the GROUP BY?

Answer 1

To start with, using a default date as you have is an extremely bad idea. You should already have an idea of that, because now you're stuck coding against a hard-coded date in some scenarios. When another developer is coding against the database (or even you at a later date) they now have to both know and remember that they have to code for exceptions based on some hard-coded date.
Further, your ownership_end_date should always be after the ownership_start_date, which this "default" date is now going to violate. If you don't know the date or the date doesn't exist yet then it should be NULL - that's exactly what NULL is for - unknown.
For your specific issue, you can do this with a LEFT JOIN that checks for other owners that fit the criteria and excludes the row if a better one exists. You didn't provide all of your table structures and it was a little unclear on whether or not you just wanted the latest owner before the sighted date (what I've done) or all owners who owned the car after the sighted date, so I don't know if this works, but something like this:
SELECT
VS.sighting_id -- ALWAYS use table aliases or prefixes for clarity
FROM
vehicle_sightings VS
INNER JOIN vehicle_vrn VRN ON VRN.vrnno = VS.plate
INNER JOIN vehicle_ownership VO ON VO.fk_sysno = VRN.fk_sysno
LEFT OUTER JOIN vehicle_ownership VO2 ON
VO2.fk_sysno = VRN.fk_sysno AND
VO2.ownership_start_date <= VS.seenDate AND
(
VO2.ownership_end_date >= VS.seenDate OR
VO2.ownership_end_date IS NULL OR
VO2.ownership_end_date = '0001-01-01 00:00:00'
) AND
VO2.ownership_start_date > VO.ownership_start_date
WHERE
VS.seenDate >= VO.ownership_start_date AND
(
VS.seenDate <= VO.ownership_end_date OR
VO.ownership_end_date IS NULL OR
VO.ownership_end_date = '0001-01-01 00:00:00'
) AND
VO2.id IS NULL -- Or some other non-nullable column
One last caveat: decide on a naming convention and stick to it (seenDate vs ownership_end_date for example) and use names that make sense (what is an fk_sysno??)

Answer 2

Here is a solution which doesn't require a subquery. It ensures that there exists no ownership records greater than the record which is returned.
SELECT
sighting_id
FROM
vehicle_sightings
INNER JOIN
vehicle_vrn ON vehicle_sightings.plate = vehicle_vrn.vrnno
INNER JOIN
vehicle_ownership ON vehicle_vrn.fk_sysno = vehicle_ownership.fk_sysno
LEFT OUTER JOIN
vehicle_ownership vo2 ON vo2.fk_sysno = vehicle_ownership.fk_sysno
AND vo2.ownership_start_date > vehicle_ownership.ownership_start_date
WHERE
vehicle_sightings.seenDate >= vehicle_ownership.ownership_start_date
AND (vehicle_sightings.seenDate <= vehicle_ownership.ownership_end_date
OR vehicle_ownership.ownership_end_date IS NULL
OR vehicle_ownership.ownership_end_date = '0001-01-01 00:00:00')
AND vo2.fk_sysno IS NULL