空值环境下含不确定及可能信息关系数据库的查询

本文以［１］中的扩展关系模型为基础在两种元组级的不完全信息──不确定及可能信息中引入属性级的不完全信息空值，使两种不同性质的不完全信息同时出现在同一关系中。为了能够查询到不同种类及不同确定程度的信息，文中制定了这种扩展关系模型上关系的查询策略，定义了能够体现这种策略的最小关系代数运算。

Null value is introduced to the extended relational model of paper[1],and two incomplete informations of different kinds simultaneously appear in a relation.The policy of querying the relation containing indefinite and maybe information under null value circumstances is given in order to obtain the information of different nature and differently definite degree,and the fundamental operations in relational algebra embodying query policy are defined.