# Relational calculus

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Relational calculus

Relational calculus consist of two calculi, the tuple relational calculus and the domain relational calculus, that are part of the relational model for databases and provide a declarative way to specify database queries. This in contrast to the relational algebra which is also part of the relational model but provides a more procedural way for specifying queries.

The relational algebra might suggest these steps to retrieve the phone numbers and names of book stores that supply "Some Sample Book":

# Join books and titles over the BookstoreID.
# Restrict the result of that join to tuples for the book "Some Sample Book".
# Project the result of that restriction over StoreName and StorePhone.

The relational calculus would formulate a descriptive, declarative way:

:Get StoreName and StorePhone for supplies such that there exists a title BK with the same BookstoreID value and with a BookTitle value of "Some Sample Book".

The relational algebra and the relational calculus are essentially logically equivalent: for any algebraic expression, there is an equivalent expression in the calculus, and vice versa. This result is known as Codd's theorem.

References

*

ee also

* The Third Manifesto
* Tutorial D
* D (data language specification)
* D4 (programming language) (an implementation of D)
* Aldat Relational Algebra

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