# Relational calculus

**Relational calculus**consist of two calculi, thetuple relational calculus and thedomain relational calculus , that are part of therelational model for databases and provide a declarative way to specify database queries. This in contrast to therelational 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

*Wikimedia Foundation.
2010.*

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