- 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:
- 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
- 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.