Imprecise schema: a rationale for relations with embedded subrelations

Exceptional conditions are anomalous data which meet the intent of a schema but not the schema definition, represent a small proportion of the database extension, and may become known only after the schema is in use. Admission of exceptional conditions is argued to suggest a representation that locally stretches the schema definition by use of relations with embedded subrelations. Attempted normalization of these relations to 1NF does not yield the static schema typically associated with such transformations. A class of relations, termed Exceptional Condition Nested Form (ECNF), is defined which allows the necessary representation of exceptional conditions while containing sufficient restrictions to prevent arbitrary and chaotic inclusion of embedded subrelations. Queries on a subset of exceptional conditions, the exceptional constraints, are provided an interpretation via an algorithm that transforms ECNF relations into 1NF relations containing two types of null values. Extensions of relational algebraic operators, suitable for interactive query navigation, are defined for use with ECNF relations containing all forms of exceptional conditions.

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