[paulsmith]

One thing I just recalled is that if you maintain a small exceptions lookup table (i.e. the ones that span state boundaries), you can use ZIP Codes as a way to uniquely look up a county name.

[graywh]

ZIP codes also span county boundaries that aren't state boundaries -- I know of several in my county alone

[mulmen]

Why would you do this?

[paulsmith]

For example, health care plans in the US are county-specific with regard to premiums, co-pays, etc. (based on demographics). Allowing someone to type in their ZIP Code to get started can be a better user experience than having them pick their county.

https://www.healthcare.gov/see-plans/