Mysql query help

[Akamaro]

Hi all, i hope you can help me, see i made a script. I got everything right so far, its a shuttle booking script. See wat it does is, client can select from where to where they wanna go, and time and date. Since it uses a table based system, it tells them the price.... just one problem, i forgot to make it that if the time and date is in the database already, client must choose a other time slot. Should i use javascript for this? Or can anyone direct me to an example? I wanna make it that i for example have 5 cars, so if 5 cars and set for the same time slot, then it must let 6th client e for the same time slot know that its full please select other one. I just need example. Thanks so much!