单选

●关系数据库设计理论主要包括3方面内容，其中起核心作用的是 (25) 。在关系模型的完整性约束中，实体完整性规则是指关系中 (26) 。引用完整性规则要求 (27) 。
(25)

A．范式 &n
bsp; B．数据模式
C．数据依赖
D．范式和数据依赖 (26) A．不允许有主行
B．属性值不允许为空
C．主键值不允许为空
D．外键值不允许为空 (27) A．不允许引用不存在的元组
B．允许引用不存在的元组
C．不允许引用不存在的属性
D．允许引用不存在的属性

试题出自试卷《2011年程序员考试考前密卷（七）-上午试题》

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