Importing data in to mysql with some control

I am trying to write a import script for loading data into an existing mysql table. I want to show the import data on the screen and let the user select the column number to go with which mysql field. Now some of the fields are mandatory like firstname lastname email address and so on........... But some of the fields might be custom fields these would be stored in a different table and the mysql fields would be custom1 custom2 ......... So the user would select what column go's with which field. No I know that some of this would have to be done server side. So I was thinking of using Ajax or just JavaScript. Does anybody know of any examples that show something like this being done. The only examples I have seen just shows data being imported into the database form csv file with out choosing where the data should go. It has to be in order when you run the script. Any ideas on where to look and how to get this started.