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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The itemNumber column provides a unique identity for each item (or row) and has an attached column-level constraint that enforces a valid value to always be supplied (NOT NULL).

itemNumber列提供了每个商品（或列）的惟一标识，它上面还有列级约束，强制要求提供正确的值（NOT NULL）。

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The "name" column will be a CHAR (30), set to NOT NULL, and have a unique index on it.

“name”列的类型是CHAR(30)，不允许为空，而且根据此列建立了一个索引。

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When a unique index is created on a column in Oracle 10g, it ensures that the NOT NULL values entered in that column are unique.

当在Oracle10g中的列上创建UNIQUE索引时，会确保此列中输入的NOT null值是惟一的。

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For primary and unique keys, only the "not null" columns are listed and can be selected to define these keys. To ease the process of defining foreign keys, the following are provided.

对于主键和惟一键，只列出“非null”列并且可以选择它们定义这些键。

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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.

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。

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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.

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。

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