摘要
本文讨论了一类特殊的Armstrong关系——不含非平凡函数依赖或多值依赖的关系,给出了这类关系的判定条件,得到了这类关系的势的下界值。并使用基于关系的投影运算方法,得到了精确的下界值,同时还涉及到了多值依赖的情形。
In the theory of relational database,a relation which satisfies precisely a given set of functional dependencies and its clsure is called as Armstrong relation.In references,C.Beeri and R.Fagin et al.Have investigated the minimal Armstrong relation,and have obtained the estimation of their cardinalities.This paper discusses a special kind of Armstrong relationsrelations without any nontrivial functional dependencies or multivalued dependencies,and gives several judgement rules for these relations,and obtains the low limit of relation's cardinality.This paper not only uses a distinct method from references,but also shows the accurate low litmit value,and further more,deals with the multivalued depencencies.
出处
《计算技术与自动化》
2002年第4期6-10,共5页
Computing Technology and Automation
关键词
函数依赖
多值依赖
数据库系统
关系数据库
relational database
functional dependency
multivalued dependency
cardinality
