一个空单元没有与单元的键相关联的值。
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。
For primary and unique keys, only the not null columns are listed and can be selected to define these keys.
约束支持主键、外键、惟一和非空约束。
Constraints Supports, primary, foreign, unique, not null constraint.
由于没有在数据库中应用键约束，必须包含信息的字段却是空的。
By not applying key constraints in the database, a field that requires information has been left empty.
在表1中，与键(000002,contact：http,t4)关联的单元为空。
In Table 1, the cell associated with the key (000002, contact: HTTP, t4) is empty.
类的新实例，该实例为空且具有指定的初始容量，并为键类型使用默认的相等比较器。
Class that is empty, has the specified initial capacity, and USES the default equality comparer for the key type.
未能启用约束。一行或多行中包含违反非空、唯一或外键约束的值。
Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints.
若要开始一个新段或空一段，可以按回车键。
To begin a new paragraph or leave a blank line, you press the Enter key.
当一个变化是一个关系，相关的外键的属性设置为空值。
When a change is made to a relationship, the related foreign-key property is set to a null value.
当一个变化是一个关系，相关的外键的属性设置为空值。
When a change is made to a relationship, the related foreign-key property is set to a null value.
应用推荐