D is a set of requirements for what Christopher J. Date and Hugh Darwen believe a relational database query language ought to be like. It is proposed in their book The Third Manifesto.
D by itself is an abstract language specification. It does not specify language syntax. Instead, it specifies desirable and undesirable language characteristics in terms of prescriptions and proscriptions. Thus, D is not a language but a family of both implemented and future languages. A "valid D" must have a certain set of features, and exclude a different set of features which Date and Darwen consider unwise. A valid D may have additional features which are outside the scope of relational databases.
Tutorial D is a specific D which is defined and used for illustration in The Third Manifesto. Implementations of D need not have the same syntax as Tutorial D. The purpose of Tutorial D is both educational and to show what a D might be like. Rel is an implementation of Tutorial D.
D s first implementation is D4, written in C#. D4 is the flagship language of Alphora's Dataphor. Others include Rel (see above), Opus, Duro, and Dee.
- Christopher J. Date and Hugh Darwen. (1998) Foundations for Future Databases: The Third Manifesto.
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