# 16.10 - Relationship Theory - Teradata Database

Product
Release Number
16.10
Release Date
June 2017
Content Type
User Guide
Publication ID
B035-1094-161K
Language
English (United States)

## 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 …
1 Unary
2 Binary
3 Tertiary
n n-ary

## 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:

• 0
• 1
• Many

## 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.