lammert - 10:05 am on Nov 29, 2010 (gmt 0)
Do you calculate the distance between the zip codes in a scripting language (PHP, Perl, ASP, etc) or with an expression in your SQL query? If it is an expression in your SQL query, you should be able to sort on the distance in one step, without either creating or updating a distance field in the database.
Updating the distance parameter in the business table may be dangerous, if two visitors enter a search query for a specific zip code at almost the same time. The two queries might overwrite each others distance values, returning wrong distance results to one or both visitors.