The cardinality of a relationship membership determines the number of entities on one side of the relationship that is associated with a single entity on the other side.
The cardinality of each relationship membership is one of the following:
Only three combinations of relationship membership cardinality can exist:
Each entity participating in the relationship can be related to only one entity of the other entity type.
A single entity of one entity type participating in the relationship can be related to one or more entities of the other type, but not conversely.
A single entity of either entity type participating in the relationship can be related to one or more entities of the other entity type.
On an Entity Relationship Diagram, at either end of the relationship line, the cardinality of the relationship membership is depicted as follows:
For example, the following illustration depicts a 1:M relationship.
The illustration shows that each customer can place one or more orders, but that only one customer places each order.
The most common cardinality for relationships is 1:M. Although 1:1 and M:N relationships sometimes occur naturally, they often appear as the result of inaccurate analysis.
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