Nested Relational
Nested RelationalAlso called MultiValue (on this site) and NonFirstNormalForm? (NF2) elsewhere.
The NestedRelational DataModel is based on the RelationalModel but relaxes the FirstNormalForm constraint requiring all values to be atomic. In NestedRelational, all values can also be relations themselves.
Although it has generally been considered an 'improper' variant of the RelationalModel, NestedRelational may have advantages for storing and integrating SemanticWeb type RaggedData? from multiple sources. Compare with ExtensibleMarkupLanguage (DocumentObjectModel), JavaScriptObjectNotation, and ResourceDescriptionFramework (also the old CodaSyl? NavigationalDatabase standard), all of which allow hierarchical structures.
A possible advantage of NestedRelational over a more ad-hoc hierarchical DataModel is that many of the mathematical analyses of the RelationalModel may apply, leading to a form more natural to work with than strict relational, but more logically tractable than pure navigational.
One interesting historical piece of software which used NestedRelational was the KappaDatabase? subproject of the Japanese FifthGenerationComputerSystems? project, based on the KernelLanguageOne? variant of PrologLanguage. Although the FGCS project was considered a failure, there may be useful lessons which can be adopted for new database systems.
http://en.wikipedia.org/wiki/Fifth_generation_computer
http://www.icot.or.jp/ARCHIVE/Museum/SOFTWARE/KAPPA/outline-E.html
http://www.icot.or.jp/ARCHIVE/Museum/SOFTWARE/KAPPA/pdst-E/node8.html
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