Simple MySQL query using SELF JOIN - mysql

Question

I have a table called image which is a table of images taken by a camera which records a car registration under variable reg, the camera number it was taken on under variable camera and a timestamp in the form yyyy-mm-dd 00:00:00 under variable whn. I have been asked to find the following:
"For each of the vehicles caught by camera 19 - show the registration, the earliest time at camera 19 and the time and camera at which it left the zone."
Therefore, I am finding the minimum time any particular cars were captured by camera 19, and then then the latest time on that date each car was captured along with the camera it was captured at. So far, I have the following code:
SELECT early.reg,
LEFT(MIN(early.whn), 10) AS date,
RIGHT(MIN(early.whn), 8) AS 'in',
RIGHT(MAX(late.whn), 8) AS 'out'
FROM image late
JOIN image early ON (early.reg = late.reg)
WHERE (early.camera = 19)
GROUP BY early.reg
This works perfectly fine, I just need to add the camera the maximum time was captured at where the max time is given by RIGHT(MAX(late.whn), 8) AS 'out' and I am struggling to do it. I tried adding late.camera within the SELECT call but then obviously you have to add GROUP BY late.camera which returns the latest time it was captured at each camera. Any help appreciated.

Answer 1

OK, Now a better understanding and clarified explanation to make sure I get what you want... The impression I am getting is as follows:
You are monitoring traffic, such as on toll roads. There are different cameras along the route both north/south or east/west bound. On any given date, you have a list of all recorded transactions read by all cameras along the route. You want to know that for any car that passed a specific camera (and we don't know the directional basis of it), you want to know where did the car finally get out of camera ranges on that route. Ex: Cameras 1-30. You know about cars getting within sight of camera 19, but they may get off the road after camera 22, 26, 29, whatever. So, for those cars that were seen at Camera 19, where was the LAST camera they passed.
If this is correct, I was close on the intent. The inner query was ALMOST the same. For a given vehicle registration ID, I am still storing the minimum and maximum dates it was spotted (could have gotten on the road at camera 4 for example, and off at camera 27 assuming cameras are sequential, but not required). The HAVING clause based on that requires that camera 19 WAS ONE of the cameras that were included in the trip. If someone got on at camera 1 and off at camera 18, they would NOT be included (provided cameras are truly sequential, but more for following along purposes).
So now I have all registrations, min and max date/times. Now, I am re-joining to the same image table based on the registration and respective min or max date since it will only be a single record per registration, no need for group by at the outer level. You would never have duplicate times for a given camera and it had to exist from the PQ query.
Now, just pull the respective camera. So the query below actually gives both the camera they were FIRST identified at, qualified as passing camera 19, and where they were LAST identified by camera.
SELECT
PQ.Reg,
LEFT(PQ.MinDateForDay, 10) AS date,
RIGHT(PQ.MinDateForDay, 8) AS 'in',
iMinDateCam.Camera CameraIn,
PQ.TimeAtCamera19,
RIGHT(PQ.MaxDateForDay, 8) AS 'out',
iMaxDateCam.Camera CameraOut
from
( SELECT
i.reg,
min( i.whn ) as MinDateForDay,
MAX( case when i.Camera = 19 then i.whn else '' end ) TimeAtCamera19,
max( i.whn ) as MaxDateForDay
FROM
image i
GROUP BY
i.reg
having
MAX( case when i.Camera = 19 then 1 else 0 end ) = 1 ) PQ
join image iMinDateCam
ON PQ.reg = iMinDateCam.reg
AND PQ.MinDateForDay = iMinDateCam.whn
join image iMaxDateCam
ON PQ.reg = iMaxDateCam.reg
AND PQ.MaxDateForDay = iMaxDateCam.whn

Answer 2

You are nearly there with the extra groupby required, just add a where clause to restrict the late.whn returned only to the max one
SELECT early.reg,
LEFT(MIN(early.whn), 10) AS date,
RIGHT(MIN(early.whn), 8) AS 'in',
RIGHT(MAX(late.whn), 8) AS 'out',
late.camera
FROM image late
JOIN image early ON (early.reg = late.reg)
WHERE (early.camera = 19)
and (late.whn = (select max(whn) from late))
GROUP BY early.reg, late.camera