Technically, the many-to-one relationship between ZIP code and "city" does not hold if the latter refers to a geopolitical entity (c.f., e.g., the definition of "city" used by the U.S. Census Bureau) rather than the region within the geographic boundaries of one or more ZIP codes (which is the USPS definition).
Technically, the many-to-one relationship between ZIP code and "city" does not hold if the latter refers to a geopolitical entity (c.f., e.g., the definition of "city" used by the U.S. Census Bureau) rather than the region within the geographic boundaries of one or more ZIP codes (which is the USPS definition).