Relational databases are based almost entirely upon set theory.
Weirdly (and lending credence to the theory that the relational model was created by some lesser demon), the relational calculus is based on sets but SQL uses bags. A table is a bag of tuples: it can contain the same object more than once.
Weirdly (and lending credence to the theory that the relational model was created by some lesser demon), the relational calculus is based on sets but SQL uses bags. A table is a bag of tuples: it can contain the same object more than once.