SQL外联接及其性质

THE OUT-JOIN IN SQL AND ITS PROPERTIES

以 ORACE的 SQL * PL US所实现的外联接运算入手 ,给出一种简单法则判定 SQL语言中外联接符号“(+)”的使用的正确性 ;进而研究了外联接作为关系代数运算的基本性质 ,从而为正确使用外联接提供依据 .

The paper provides a single role to decide wether a where-condition of a SQL statment,which includes out joins and has immplimented by Oracle,is correct in syntax.In addition,some algebraic properties of out joins have been studied.Among them are associateive law of out joins,distributive law of out join with unions,commutative law with projects,etc.