A business rule indicating the number of times a particular object or activity may occur.
Cardinality of a Dependency
The number of executions of each process that may occur before or after each execution of the other process.
Cardinality of a Subprocess
The number of times a subprocess is executed during each execution of the process of which it forms a part.
Cardinality of a Relationship Type
The number of pairings in which an entity may participate in the relationship membership.
In the example on relationship pairing and grouping, many persons may live at one address. This multiplicity is an important property of a relationship and is known as cardinality.
Example of One-To-Many Cardinality
The example on relationship pairing includes a cardinality based on the observation that "each PERSON lives at one and only one ADDRESS," and the inverse, "each ADDRESS has many PERSONs." One-to-many is the most common form of cardinality. It may be written as 1:M.
Forms of Cardinality
There are three forms of cardinality:
* One-to-one (1:1): For example, PERSON married to PERSON, where a PERSON may be married to one and only one other PERSON and vice versa.
* One-to-many (1:M): For example, BOOK printed as EDITION, where each BOOK may be printed in several EDITIONs, but each EDITION is a printing of a single BOOK.
* Many-to-many (M:N): For example, AUTHOR writes BOOK, where any AUTHOR can write many BOOKs, and any BOOK can be written by many AUTHORs.
One-to-Many Relationship
Given any grouping under a one-to-many relationship, there will be one entity paired with one or more entities of the same or another type. For the entities with a "one" membership (e.g., ORDER placed by one CUSTOMER), there is a single, identifiable, related entity (e.g., the customer who placed the order). For entities with a "many" membership, however, there are no easily identifiable related entities.
Example of one to many relationship
Consider the relationship "CUSTOMER may place one or more ORDERs." From the viewpoint of an order, the customer who placed it is easily distinguished because there is only one of them. It is not so simple to distinguish a particular ORDER for a given CUSTOMER, however. You need to use some method for distinguishing any one ORDER from all other ORDERs for that CUSTOMER, because there may be many orders. Similarly, if ORDER has a one-to-many relationship to ORDER ITEM, for each ORDER ITEM there is a single ORDER and a single CUSTOMER. For each CUSTOMER, however, there is no one uniquely identifiable ORDER or ORDER ITEM.
Fixed Cardinality
A type of cardinality condition in which a cardinality of a dependency, relationship or subprocess is always the same number.
A relationship may have fixed cardinality (e.g., BUDGET consists of twelve PERIODs, or CHILD is born to two PARENTs). With fixed cardinality, the number of entities participating in each grouping is known for at least one of the entity types.
Cardinality Condition
A type of predicate condition that places constraints on the cardinality of a dependency, relationship or subprocess.
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