gvoshol 73146 (8/19/2016)
Have you considered what you will do with zip codes that cross county lines? Does your data source (when you decide on one) split the zip code by county?Consider 49423, Holland, MI. The city of Holland straddles the Ottawa/Allegan county line, with most of the city population in Ottawa. But 49423 extends farther south into Allegan outside the city limits. I'd guess that the Allegan portion of the zip code is the most populous in the county, even excluding the Ottawa part of the zip code. But if you look up 49423, most sources would list it as being in Ottawa, and you'd miss it entirely in Allegan.
The possibility did cross my mind once, but I'd forgotten about it until you brought it up. The table involved doesn't have a unique constraint on the zip code column, so I think we'll be OK.
Kindest Regards, Rod Connect with me on LinkedIn.