Match one out of several possibilities

Hello,
I'm looking for a formula that will return a zip code when looking up a county. Obviously, counties may have several zip codes within them, but I would like Excel to pick one randomly and match it to the county that it's in. I have a table with all zip codes and their associated counties.
Then I have an other table with people and the counties they live in. I want to match any zip code to them (regardless of what it truly is) that is in that county...
Thank you!
Caroline