Abstract
This paper presents a precise definition of basic relational notions as well as a precise and general definition of the relational algebra. The paper also illustrates a method for developing semi-formal definitions of data models. A nearly formal definition of relations with unordered attributes is given, and algebraic operations are described as operating on and producing relations thus defined. The definition of algebric operations contains, as a special case, the "classical" definition of the algebra (see e.g. [CODD72]). But, in addition, several operations are generalized. The closure of the algebra is characterized precisely, with the help of an operation for renaming relation attributes.
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