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。
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。
Constraints Supports, primary, foreign, unique, not null constraint.
约束支持主键、外键、惟一和非空约束。
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。
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。
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。
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。
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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