> A client of mine, a rail car storage company, has asked that I create a
> PHP/MySQL application in which they will maintain and track rail cars. I
> having a bit of trouble however working through one of thier requirements.
> They need to know in what position the rail car is on each track. For
> example, they might have a track called X which will hold 30 rail cars.
> would enter the car information for 30 cars and associate each of them
> track X. If one of the car owners decides to move car 3 out of storage,
> the remaining 29 cars must be re-numbered ( ie; car 1 and 2 would remain
> car 4-30 would become car 3-29 ). In the same manner, I need to be able to
> add a car to the track once an empty slot is available. For example, If
> new car goes at the front of the track then it would become car 1 and the
> remaining cars, originally numbered 1-29 would become 2-30.

Not sure if this helps or if you already realize this...

Say you have a table that identifies the track_id and car_id. Now, you
delete the car_id for car #3, like you've said. So, that row is deleted and
you've got a hole now. You can run a query such as:

UPDATE table SET car_id = car_id - 1 WHERE car_id BETWEEN 4 AND 30 AND
track_id = XX

to adjust all of the other car_id numbers.

Now, say you want to add a new car to the beginning.

UPDATE table SET car_id = car_id + 1 WHERE track_id = XX

and then insert your new car at position #1.

Throw in some checks to make sure you don't go over 30 cars and you should
have it. You can get a count of how many cars are on a certain track with:

SELECT COUNT(*) AS c FROM table WHERE track_id = XX

Hope that helps. It sounds like a fun project.

---John Holmes...

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