Relational calculus consist of two calculi, the
tuple relational calculusand the domain relational calculus, that are part of the relational modelfor databases and provide a declarative way to specify database queries. This in contrast to the relational algebrawhich 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
The Third Manifesto
D (data language specification)
D4 (programming language)(an implementation of D)
Aldat Relational Algebra
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