【摘要】 文章在深刻理解空值语义的基础上 ,给出了一种处理占位型空值的方法。讨论了空值环境下关系数据库的查询处理策略及程序实现 ,定义了含三种查询操作的关系代数最小完备集中的关系代数运算 ,并对查询计算的有效性和完备性进行了扩展与分析更多还原

【Abstract】 In this paper,a method to process holdplace is proposed on the basis of the semantics of null value and the prgram design and querying policy under null value circumstances in relational database is discussed.Moreover,the fundamental operations in relational algebra with three kinds of querying operations are defined.Also,soundness and completeness in querying calculation are analysed at the end the paper.更多还原