Domain Relational Calculus - Examples

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:

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:

\begin{align} \{ {\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{align}

In this example, we're only looking for the name, and that's B. 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:

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.

Read more about this topic:  Domain Relational Calculus

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