Relational Algebras and Relational Calculus

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Suppose we have a statement $S$ such as "The number of people connected to Bob through any number of friends." Why is this statement expressible in relational algebra but not in relational calculus? Furthermore, are there statements that can be expressed in relational calculus but not in relational algebra? Is there a preference for Domain Relational Calculus versus Tuple Relational Calculus?

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Codd's Theorem (1970s) shows that relational calculi (both domain- and tuple-) and relation algebra are equally expressive. That means that there cannot be a statement expressible in relational algebra but not in relational calculi, and vice versa.