Sometimes you may want to change this order or perhaps only specify values for columns that have NOT NULL constraints.

有时候您希望更改这种顺序或可能希望只为具有NOT NULL约束的列指定值。

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This new validation will enforce NOT null column constraints by rejecting null values, but only in situations where your database would reject a null value.

这种校验会通过拒绝空值，强迫加上非空[not NULL]字段的约束，但是也只有在这种情况下，你的数据库会拒绝空值。

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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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Some constraints will not change (not null, maximum length in the database), but some will vary with the country.

一些约束不会有变化（非空、数据库中的最大长度），但有一些会因国家而不同。

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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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This mechanism has the benefit of limiting repeated conditionals testing for certain constraints, such as collection sizes or the previously discussed not-null condition.

这种机制具有限制针对特定约束条件的重复条件测试的好处，如集合大小或之前讨论过的非null的情况。

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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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