Domain calculus

From Academic Kids

In computer science, domain relational calculus is a calculus that was introduced by Edgar F. Codd as part of the relational model in order to give a declarative database query language for this data model.

A DRC Query has the form:

<math> { | p() } <math>

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 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), and we can use the existential quantifier (∃) and the universal quantifier (∀) to bind the variables.

Here are examples of DRC:

Let's take A,B1,C to mean Rank, Name, ID and D,E,F to mean Name, DeptName, ID

Find all Captains of the Enterprise:

  • {<A, B1, C> | <A, B1, C> in Enterprise ∧ A = "Captain" }

In this example, A,B1,C is the result set and a set in the table Enterprise.


Find Names of Enterprise crewmembers who are in Stellar Cartography:

  • {<B1> | ∃ A,C ( <A,B1,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 <B1> denotes the column Name. F = C is a requirement, because we need to find Enterprise Crewmembers AND they are in the Stellar Cartography Department.


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