Updating relational databases through weak instance interfaces

The problem of updating databases through interfaces based on the weak instance model is studied, thus extending previous proposals that considered them only from the query point of view. Insertions and deletions of tuples are considered. As a preliminary tool, a lattice on states is defined, based on the information content of the various states. Potential results of an insertion are states that contain at least the information in the original state and that in the new tuple. Sometimes there is no potential result, and in the other cases there may be many of them. We argue that the insertion is deterministic if the state that contains the information common to all the potential results (the greatest lower bound, in the lattice framework) is a potential result itself. Effective characterizations for the various cases exist. A symmetric approach is followed for deletions, with fewer cases, since there are always potential results; determinism is characterized as a consequence.