Re: Help with MySQL Logic

> 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 am > 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. They > would enter the car information for 30 cars and associate each of them with > track X. If one of the car owners decides to move car 3 out of storage, then > the remaining 29 cars must be re-numbered ( ie; car 1 and 2 would remain and > 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 the > 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...