Divide
 The last operation of relational algebra.
 Often overlooked.
 Never used in practice.
So why does it exist?
 In early days of the relational model, there was another language approach:
relational calculus.
 Based on predicate calculus, which has two quantifiers
 : There
exists a tuple for which a given condition is true.
 : For all
tuples, a given condition is true.
 Obvious question: which query language (approach) is better? Are
there any queries that can be expressed in one language but not the
other?
 Answer: Relational algebra and relational calculus are equally expressive.
 But this is only true if the relational algebra includes
a counterpart to ∀.
 Division is that counterpart.
