Abstract: The translating of Entity-Relationship model into Relational Data model is an important process of a database design. There are some defects in the traditional rules of translating, and these errors usually lead to a problem（called invalid-key） in a relation schema coming from the Translating. This paper provides an exhaustive analysis of the problem, and shows that time-varying of relationship is the main reason which causes an invalid-key problem. This paper develops a new property for relationship, called state, and divides relationship into dynamic and static. This paper presents new rules of translating. It eliminates the defects in traditional rules, and solves the invalid-key problem.

Key words: Entity-Relationship, invalid-key, state of relationship, dynamic relationship, rule of translating

摘要： 数据库设计的一个重要环节是将E-R模型转换为关系模型，但传统的E-R模型向关系模型的转换规则中存在一定的缺陷，在将具有时变性的多对多联系转换为关系后会导致关系的键无效。对导致转换后关系键无效的问题进行了深入分析，将问题根源归因为实体型间联系的时间性变化。在传统E-R模型引入联系的时变性质，据此将联系划分为动态联系和静态联系。提出了基于联系时变状态的新的E-R模型向关系模型的转换规则，修正了传统转换规则中的缺陷并解决了转换后的关系键失效问题。

关键词: 实体-联系（E-R）模型, 键无效, 联系状态, 动态联系, 转换规则