# Cardinality Constraints

They help us express the number of entities to which another entity can be associated via a relationship set.

They are most helpful in binary relationship sets.

For a binary relationship set, the mapping cardinality must be one of the following types:

* one to one
* one to many
* many to one
* many to many

In a ternary relationship, we usually limit the number of one-one relationships (denoted by an arrow) to 1.

If there is more than one arrow, there are two ways of defining the meaning:

A ternary relationship R between A, B and C with arrows to B and C could mean:

1. Each A entity is associated with a unique entity from B and C (or)
2. Each pair of entities from (A, B) is associated with a unique C entity, and each pair (A, C) is associated with a unique B


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