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Domain relational calculus
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<h2 id="examples">Examples</h2> <p>Let (A, B, C) mean (Rank, Name, ID) in the Enterprise relation </p><p>and let (D, E, F) mean (Name, DeptName, ID) in the Department relation </p><p>All captains of the starship <a href="/facts/USS_Enterprise_(NCC-1701)/4vx6pL1L">USS Enterprise</a>: { ⟨ A , B , C ⟩ ∣ ⟨ A , B , C ⟩ ∈ E n t e r p r i s e ∧ A = ′ C a p t a i n ′ } {\displaystyle \left\{\ {\left\langle A,B,C\right\rangle }\mid {\left\langle A,B,C\right\rangle \in \mathrm {Enterprise} \ \land \ A=\mathrm {'Captain'} }\ \right\}} </p><p>In this example, A, B, C denotes both the result set and a set in the table Enterprise. </p><p>Names of Enterprise crew members who are in Stellar Cartography: { ⟨ B ⟩ ∣ ∃ A , C ⟨ A , B , C ⟩ ∈ E n t e r p r i s e ∧ ∃ D , E , F ⟨ D , E , F ⟩ ∈ D e p a r t m e n t s ∧ F = C ∧ E = ′ S t e l l a r C a r t o g r a p h y ′ } {\displaystyle {\begin{aligned}\{{\left\langle B\right\rangle }&\mid {\exists A,C\ \left\langle A,B,C\right\rangle \in \mathrm {Enterprise} }\\&\land \ {\exists D,E,F\ \left\langle D,E,F\right\rangle \in \mathrm {Departments} }\\&\land \ F=C\\&\land \ E=\mathrm {'Stellar\ Cartography'} \}\\\end{aligned}}} </p><p>In this example, we're only looking for the name, and that's B. The condition F = C is a requirement that describes the intersection of Enterprise crew members AND members of the Stellar Cartography Department. </p><p>An alternate representation of the previous example would be: { ⟨ B ⟩ ∣ ∃ A , C ⟨ A , B , C ⟩ ∈ E n t e r p r i s e ∧ ∃ D ⟨ D , ′ S t e l l a r C a r t o g r a p h y ′ , C ⟩ ∈ D e p a r t m e n t s } {\displaystyle {\begin{aligned}\{{\left\langle B\right\rangle }&\mid {\exists A,C\ \left\langle A,B,C\right\rangle \in \mathrm {Enterprise} }\\&\land \ {\exists D\ \left\langle D,\mathrm {'Stellar\ Cartography'} ,C\right\rangle \in \mathrm {Departments} }\}\\\end{aligned}}} </p><p>In this example, the value of the requested F domain is directly placed in the formula and the C domain variable is re-used in the query for the existence of a department, since it already holds a crew member's ID. </p><p> Both of them written in <a href="/facts/SQL/UjQLJcYm">SQL</a> will be like:</p>SELECT B FROM Enterprise JOIN Department ON F = C AND E = 'Stellar Cartography'; <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Relational_calculus/HRBgfeqR">Relational calculus</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="http://des.sourceforge.net">DES – An educational tool for working with Domain Relational Calculus and other formal languages</a></li> <li><a href="https://web.archive.org/web/20100118113532/http://www.eas.asu.edu/~winrdbi/">WinRDBI – An educational tool for working with Domain Relational Calculus and other formal languages</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Michel Lacroix, Alain Pirotte: Domain-Oriented Relational Languages. VLDB 1977: 370-378 <a href="https://dl.acm.org/citation.cfm?id=1286620" target="_blank">https://dl.acm.org/citation.cfm?id=1286620</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>E. F. Codd: Relational Completeness of Data Base Sub-languages. In R. Rustin, editor, Data Base Systems. Prentice Hall, 1972 <a href="/wiki/E._F._Codd" target="_blank">/wiki/E._F._Codd</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> </ol>