Yellow Pages, Zip or Postal Code and Telephone directories.

黄页、邮编和电话名录。

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For example, the key could be a unique user ID, a stock symbol or, in our case, a zip (postal) code.

例如，主键可以是一个唯一的用户ID、一个股票符号，在我们的实例中，主键是一个邮政编码。

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One big area for expansion: presorting, or arranging mail by ZIP code, to get a break on postal rates.

FORBES: Delivery Problems