Constraints Supports, primary, foreign, unique, not null constraint.

约束 支持主键、外键、惟一和非空约束。

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For primary and unique keys, only the not null columns are listed and can be selected to define these keys.

对于主键和外键，只有非空列才能被列出和选择来定义这些键。

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You can also use the system catalog views to retrieve information about NOT NULL, unique, primary key, foreign key, and table check constraints. For example.

也可以使用系统编目视图来检索关于NOT NULL、惟一、主键、外键和表检查约束的信息。

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