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在关系数据库中，实现主键值惟一标识元组的作用是通过( )。
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关系数据库中标识元组的属性或最小属性组称为
主键
关系
分量
字段
在关系数据库中实现主键值惟一标识元组的作用是通过
在关系数据库中关于主键的正确说法是
主键只能由一个属性构成
主键值可以为空
每个关系可以有多个主键
主键在表中唯一地标识一个记录
在关系数据库中实现主键值唯一标识元组的作用是通过
实体完整性规则
参照完整性规则
用户定义完整性规则
强制不能为空值
关系数据库中实现主码标识元组的作用是通过
实体完整性规则
参照完整性规则
用户自定义的完整性
属性的值域
在关系数据库中码主键的主要作用是
描述关系表中对象的取值范围
构建模式
描述关系表中对象的取值特征
唯一标识一个元组
关系数据库中能惟一标识元组的那个属性称为
惟一性属性
不可改动的保留字段
关系元组的惟一性
主码
在关系数据库中实现主码标识元组的作用是通过______
实体完整性规则
参照完整性规则
用户自定义的完整性
属性的值域
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