 Databases

Solutions to all three questions of the examination dated 3 June 2015. The cover permits answers to two questions.^[ref]

Use classical relational algebra with set semantics. Write $C,S,U,P$ for the paper's Car, Sales, Customer and Salesperson database relations. A natural join between $C$ and $S$ matches serialnumber; joins involving a salesperson explicitly match salespersonid to empid. Identifiers, rather than names, distinguish customers and salespeople. The calculations use the ordinary non-null relational model; the SQL aggregate below also explains its empty-input behavior.