Relational calculus consists 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:
The relational calculus would formulate a descriptive, declarative way:
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.
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