Past exam of the computer science course of the University of Oxford / 2015 / Databases / 1 / b

The right-hand relational projection lists the models of cars for which at least one recorded sale was made by a salesperson earning strictly more than $30000$. The outer relational difference removes precisely those models from all models present in Car. Thus the answer isThis includes models with no sales at all, as well as models sold exclusively by salespeople whose salaries are at most $30000$. A model sold by both a low-paid salesperson and a high-paid salesperson is excluded. A salary exactly equal to $30000$ does not trigger exclusion. These are model-level statements: one unsold car of a model does not rescue that model if a different car of the same model had a qualifying sale.