BCS 260 Lecture : BCS260W4N13
Document Summary
An item can be supplied by many suppliers and a supplier can supply many items: note the names of the relationships are telling/descriptive. Cardinality is the number of entity instances (objects) that are in the relationship and varies by the side of the relationship. Let"s look at the relation born in which represents whether a person is born in a country. We"ll assume that every person is born in a country. When we talk about the cardinality here, the answer isn"t that the cardinality is two because we have two entities on our er diagram. In the database we might have billions of entity instances of person and hundred entity instances of country . If we don"t name, what could the relationship represent: a person was born in a country, a person is a citizen of a country, a person has visited a country, a person likes the food from the country.
