在本例中，EMPNO是表的唯一键。

In this example, EMPNO is the unique key of the table.

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即它在一个唯一键组合上只会产生一行数据。

That is it only brings in one row of data for a unique key combination.

youdao

但和数据库不同的是，本地的唯一键还不行。

Unlike in the case of a database, however, a locally unique key will not do.

youdao

But, of course, what you saw in recitation, and hopefully, what you can now think very quickly by looking at this, is that this is not the only configuration of pi bonds that we could have in benzene.

当然，你们在习题课看到过，你们通过看这个可以很快的想到，这不是苯环里,唯一的π键构型。

麻省理工公开课 - 化学原理课程节选