BCS 260 Lecture Notes - Lecture 4: Relational Database
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The relationship we will look at what is homeroom teacher to. Obviously this is a relationship between homeroom teachers and. In order to figure out cardinality, we need to look at the instances that come from these entities. Is it required or optional (this is sometimes called minimum cardinality or a participation constraint) We will never ignore the maximum cardinality, though (we may ignore a specific limit on maximum cardinality, for example no more than 25 customers per class, but not whether it"s one or many) To reiterate: similar with our country database. It may be true that every country has people born in it. Nevertheless, if we intend to add a country and then the people later, we may violate some database constraint unless we bulk load the data: for these reasons, we may (in practice) ignore the minimum cardinality constraints.
