关系数据库不可用空值的查询与处理

Processing for Querying Inapplicability Null Values in Relational Databases

现有的基于关系数据模型的商业数据库采用空值对缺失信息进行建模与处理,然而,单一的空值解释无法体现空值本身的丰富语义。事实上,在相关研究中空值通常被解释为‘值未知’,‘值不可用’以及‘值不存在’等。文中主要研究不可用空值的查询与处理。通过仔细地观察和深刻地理解,分别在传统关系数据库查询和模糊数据库查询中讨论不同语义背景和查询条件下不可用空值的处理和分类。此外,还针对涉及不可用空值的传统关系数据库查询提出选择运算和差运算算法,这些算法使文中的研究更具实用性。

Null values are used to model missing information in databases, however, the rich semantics of null values cannot be reflected by a single symbol ＇NULL＇.Actually, null values have been interpreted as ＇Unknown＇,or ＇Inapplicable＇ ,or ＇Not exist＇, etc., in related researches.This paper mainly deals with the querying with such null values, especially null values representing for inapplicable information.Through carefully observation and deeply understanding,different types of inappli- cable values under different contexts and querying criterions are discussed and classified in traditional database querying and fuzzy database querying respectively. Furthermore, the selection operation and the difference operation algorithms are proposed under traditional databases＇ environment.These algorithms make the research more practical.