Then, an update is a sentence in update calculus with all abduction models expressing its semantics. The semantics also preserve all transformations valid in relation calculus. Section 3 is devoted to the expressive power of update calculus. It is shown that it is possible to specify database transformations involving transitive closure. In section 4, dynamic relations and update algebra are defined.
Dynamic relations serve as a representation of intermediate results in update evaluation.
The algebra contains various operations over dynamic relations. Section 5 includes the proof that update calculus can be reduced to update algebra and vice versa. Finally, section 6 contains a discussion of different approaches to declarative updates. Doubtless, the paper gives a worthwhile contribution to a formalization of nondeterministic and set-oriented updates. Problems and their solutions are well-structured and motivated by nontrivial examples.