空值环境下的数据依赖保持条件

HOLD CONDITIONS OF DATA DEPENDENCIES UNDER THE NULL VALUE CIRCUMSTANCES

本文提出了空值环境下的函数依赖强保持、中间保持,弱保持条件。研究了它们之间的关系。 根据空值环境下的含空值关系的非空化结果关系的数据依赖是函数依赖的充要条件这一结论,提出了新的空值环境下的多值依赖的强、弱保持条件。给出了多值依赖下强、弱保持的相应定理。

The strong hold, middle hold and weak hold conditions of Functional Dependencies under the null value circumstances are given. The relations among them are also discussed. According to the conclusion that the Data Dependencies of the result relation without null values from the relation with null values under the null value circumstances is the full and.essential condition for Functional Dependencies, the strong and weak hold of Multiple Value Dependencies under the null value circumstances is mentioned and the proper dealing with the strong and weak hold of Functional Dependeneies is discussed.