Definition of Degree
The degree of a relationship is the number of entities associated in the relationship. Typical degree descriptions are provided in the following table:
|For a relationship among this many entities …||The following term is commonly used to describe the relationship …|
Definition of Connectivity
Connectivity refers to the mapping of entity occurrences in a relationship. The possible values for the connectivity of a relationship are three:
Definition of Cardinality
With respect to E-R theory, the term cardinality refers to the integer number represented by the symbol M in a 1:M or M:M relationship. This number describes the constraint on the number of entity instances that are related through the relationship.
In this context, cardinality does not refer to the number of tuples in a relation.
Definition of Existence Dependency
Whenever the existence of an entity depends on the existence of another entity, that relationship is described as an existence dependency (ED).
For example, suppose there are two entities named Author and Book. A writer might have been provided with an advance to write a book for a publisher, but if this is the first book written by the writer for this publisher, there will be no entry in the Book entity until the contracted book has been written and published. In this case, the relationship between Author and Book is 1:0, indicating that the existence of an instance of Book is optional.