主键不允许空值。

Primary keys do not allow null values.

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定义主键以对不允许空值的指定列中输入的值强制唯一性。

Define a primary key to enforce uniqueness for values entered in specified columns that do not allow nulls.

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可将唯一约束附加到允许空值的列，但是只能将主键约束附加到不允许空值的列。

You can attach unique constraints to columns that allow null values, whereas you can attach primary key constraints only to columns that do not allow null values.

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