这些键可以是多个列的并置，通常象“社会保障号”、雇员标识、ISBN或部件标识这样的标识用于这些角色中。

These keys can be a concatenation of multiple columns, and often identifiers like Social Security Numbers, employee IDs, ISBNs, or part IDs are used in these roles.

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附注：当今社会，要想成为“老练的求知者”，还有一个法宝就是——按下“订阅键”或用电子邮件订阅“学海无涯”栏目哦。

P.S. Another practice of the sophisticated learner in modern times is to subscribe to Mission to Learn by RSS feed or by e-mail.

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