I saw a database insert sheet with zip codes for about a half dozen states and it was a huge, huge file. The guy that had it said you'd have to break it down into smaller chunks to upload it to the database. If this was just for a few states I can't image what it would be like for the entire country.
It would probably take a week to get it all in the database. Not only that but they often change the zip code areas and add to them, so you'd have to stay on top of that too.