A relational programming language (RPL) is a DeclarativeLanguage built around the RelationalModel of data.
StructuredQueryLanguage (SQL) is an example.

A relation can be viewed as a reversible function that can return an arbitrary number of values. For example, the "square root" relation maps the number [0] to itself, but maps [4] to [2 -2]. Relations can also be arranged as tables, and can be self-describing. For these reasons, relational programming can be seen as a generalization of functional programming. The internet archive has a fascinating set of papers from the 1980's by Bruce J MacLennan? at the Naval Postgraduate school in Monterey:

*And here I was thinking about stored procedure / trigger languages that execute "in" a RelationalDatabase:*

See also Jay Earley's VERS 2, a ProceduralProgrammingLanguage? built around the RelationalModel of data. For example, this paper:

Also LIBRA - A general-purpose programming language based on the algebra of binary relations http://www.cs.adelaide.edu.au/~dwyer/TR95-10_TOC.html

The PrologLanguage could be considered a relational programming language.*Not really. PrologLanguage has no inherent support for labeled tuples. All it has is untyped lists of "atoms". In this respect it is quite similar to LispLanguage. I suppose you could add support for labeled (typed) tuples, though. OTOH one of the fundaments of PrologLanguage is an in-memory database of "facts" (forgot the actual term - clauses maybe? - but "facts" describes them better than original).*
PrologLanguage has tuples, but you examine them using pattern matching instead of using labels. Prolog is based on predicate logic rather than relational calculus. Relational calculus is I think a restricted form of predicate logic.
The truth statements of prologs are similar to the rows of a relational table. (are there your tuples above?)
*rules* of Prolog. It is straightforward to have a "directReports" relationship in a relational table--associating each employee with his/her boss--but deriving the "indirectReports" relationship (the transitive closure of the directReports relationship) is trickier. You can do it with RelationalJoin?s, but that's ugly. A rule in PrologLanguage expresses this relationship much more succinctly.
*This can be made into a PrologForMassiveData.*

Also see TutorialDee, RelationalLanguage, PrologForMassiveData, EmbraceSql

A relation can be viewed as a reversible function that can return an arbitrary number of values. For example, the "square root" relation maps the number [0] to itself, but maps [4] to [2 -2]. Relations can also be arranged as tables, and can be self-describing. For these reasons, relational programming can be seen as a generalization of functional programming. The internet archive has a fascinating set of papers from the 1980's by Bruce J MacLennan? at the Naval Postgraduate school in Monterey:

- Introduction to Relational Programming (Jun 1981) - https://archive.org/details/introductiontore00macl
- Overview of relational programming (Nov 1981) - https://archive.org/details/overviewofrelati00macl
- A Relational Program for a Syntax Directed Editor (Apr 1982) - https://archive.org/details/relationalprogra00macl
- Relational Programming (Sep 1983) - https://archive.org/details/relationalprogra83012macl
- Four Relational Programs (Nov 1986) - https://archive.org/details/fourrelationalpr00macl

- TransactSQL - Sybase & SQL Server
- PL/SQL - Oracle

See also Jay Earley's VERS 2, a ProceduralProgrammingLanguage? built around the RelationalModel of data. For example, this paper:

- Jay Earley
- "Relational Level Data Structures for Programming Languages". Acta Informatica 2: 293-309 (1973)

Also LIBRA - A general-purpose programming language based on the algebra of binary relations http://www.cs.adelaide.edu.au/~dwyer/TR95-10_TOC.html

- "Ordinary programming languages calculate functions. Sometimes a function is inappropriate. For example, 4 has two square roots, +2 and -2, but an ordinary programming language provides a sqrt function that returns only one of the roots."

The PrologLanguage could be considered a relational programming language.

employee ('Joe Doe', 1979, 'Department of Defense'). employee_managed_by ('Joe Doe', 'Mister X').The thing that RelationalCalculus is missing are the

Also see TutorialDee, RelationalLanguage, PrologForMassiveData, EmbraceSql

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