database(relation(name("Addresses"),
                  header(attribute("ID") → type(int)×PrimaryKey,
                         attribute("city") → type(char),
                         attribute("state") → type(char)),
                  data(tuple("ID" → 18, "city" → "Cambridge", "state" → "MA")))
         relation(name("People"),
                  header(attribute("ID") → type(int)×PrimaryKey,
                         attribute("fname") → type(char),
                         attribute("addr") → type(int)×ForeignKey("Adresses", "ID")),
                  data(tuple("ID" → 7, "fname" → "bob", "addr" → 18),
                       tuple("ID" → 8, "fname" → "sue", "addr" → null))))