如何在下一个免费地点添加项目

I have a program that lets a user add a customer, name, and an RD number (the RD number is similar to an ID) I have an import script that uploads already existing details to my database, but there are free slots where the RD numbers should be (meaning that some customers have high RD numbers, some have low, some have inbetween. They are not all filled in order.) I need to set my program so that when I add a customer, the next available RD number is set by default.I have a program that lets a user add a custome