Because a city/region/state can be uniquely identified with a postal code (hell, in Ireland, the entire address is encapsulated in the postal code), but the reverse is not true.
At scale, repeated low-cardinality columns matter a great deal.
At scale, repeated low-cardinality columns matter a great deal.
SELECT zipcode.rural_urban_code, COUNT(*) AS n_customer FROM customer INNER JOIN zipcode USING(zipcode) GROUP BY 1;