Relational Database Design and Modeling for Software Engineers
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Cardinality and Participation

You have drawn Student, Course, and a diamond labelled "enrols in". The picture is still missing the two facts that decide your tables.

How many courses can one student enrol in? And must every student enrol in at least one?

The first question is cardinality. The second is participation. Together with degree, they are what turn a drawing into a schema.

Degree: how many entity types take part

The degree of a relationship is the number of entity types connected to it.

Unary, degree 1

A unary relationship connects an entity type to itself. It is also called recursive.

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