Domain relational calculus
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In computer science, domain relational calculus (DRC) is a calculus that was introduced by Michel Lacroix and Alain Pirotte as a declarative database query language for the relational data model [1].
In DRC, queries have the form:
- < X1,X2,....,Xn > | p( < X1,X2,....,Xn > )
where each Xi is either a domain variable or constant, and p(<X1, X2, ...., Xn>) denotes a DRC formula. The result of the query is the set of tuples Xi to Xn which makes the DRC formula true.
This language uses the same operators as tuple calculus; Logicial operators ∧ (and), ∨ (or) and ¬ (not). The existential quantifier (∃) and the universal quantifier (∀) can be used to bind the variables.
Its computational expresivity is equivalent to that of Relational algebra [2]. .
[edit] Examples
Let A, B, C mean Rank, Name, ID and D, E, F to mean Name, DeptName, ID
Find all captains of the starship USS Enterprise:
- {<A, B, C> | <A, B, C> in Enterprise ∧ A = "Captain" }
In this example, A, B, C denotes both the result set and a set in the table Enterprise.
Find Names of Enterprise crewmembers who are in Stellar Cartography:
- {<B> | ∃ A, C ( <A, B, C> in Enterprise ∧ ∃ D, E, F(<D, E, F> in Departments ∧ F = C ∧ E = "Stellar Cartography" ))}
In this example, we're only looking for the name, so <B> denotes the column Name. F = C is a requirement, because we need to find Enterprise crew members AND they are in the Stellar Cartography Department.
An alternate representation of the previous example would be:
- {<B> | ∃ A, C (<A, B, C> in Enterprise ∧ ∃ D (<D, "Stellar Cartography", C> in Departments))}
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.
