Can we use the universal instance assumption without using nulls?
Yehoshua Sagiv · 1981
We claim that the representative instance of [Ho1, Va3] is a correct representation of the data stored in a database even when the relations of the database are not the projections of a single universal instance. If no constraint (other than functional and join dependencies) is imposed on the data, then projections of the representative instance cannot always be computed by lossless joins. We show that if the database satisfies a modified foreign-key constraint, then projections of the representative instance can be computed by performing the union of several lossless joins. A class of relation schemes for which no constraint is necessary is characterized, and we show how to compute projections of the representative instance for databases that belong to this class.