A reason of relevance to the enterprise why entities from one or from two entity types are considered as a logical unit. A relationship type is often referred to simply as a relationship.
Relationship membership is the participation of an entity type in a relationship.
The associations between its entity types are as important to a business as the entities, themselves. These are analyzed as relationship types. The occurrence of a pairing is a relevant association between two entities. For example, Ian Palmer "writes" Database Management, where "writes" associates an AUTHOR entity type with a BOOK entity type. The relationship type is named with the verb (e.g., "writes") that states the reason why entities of the two types may be associated.
A relationship type can be seen from the point of view of each of the two participating entity types. The name is different, depending on which entity type is being considered.
For example, the "writes" relationship is seen from:
* The Author's viewpoint as: AUTHOR "writes" BOOK (active name)
* The Book's viewpoint as: BOOK "is written by" AUTHOR (passive name)
These two views are called the relationship memberships. Each relationship pairing always has two memberships.
Relationship Viewpoints
In the above example, you have taken the viewpoint of each entity type and considered whether it is an active or passive participant in the relationship.
Each of these viewpoints is a relationship membership, and each relationship pairing has two of them. A relationship type often comes about because there is some process that associates the entity types (e.g., WRITE BOOK). If there is such a process, the name of the relationship type will come from it. In this case, the active participant is the entity type that carries out the process (e.g., it is the AUTHOR who writes the BOOK). Similarly, the passive participant is the entity type that is the object of the process (e.g., the BOOK that is written).
Relationship Action or Pairing Action
A type of action on one or more pairings of a given relationship by an elementary process following an action on an entity.
Relationship Cardinality
The number of times an entity may participate in a membership pairing with another entity or entities. For instance a person may be the "parent of" zero, one or many other persons.
Membership or Relationship Optionality
Membership of an entity type in a relationship, such that entities of the type may exist without participating in a pairing under the relationship.
When describing a business, determine whether or not an entity of a given type always participates in a pairing of a given relationship. For example, an EDITION is always of a BOOK and cannot exist unless it is associated with a BOOK. The relationship membership "EDITION is printing of BOOK" is therefore said to be mandatory.
You may, however, find that at some time there are BOOKs that have not yet been printed as an EDITION. The relationship membership "BOOK printed as EDITION" is therefore said to be optional.
Optionality of Each Relationship Membership
The state of being mandatory or optional is a property not of the whole relationship, but of each of the two relationship memberships. Optionality of a relationship considers whether entities of a given entity type can exist without participating in any pairing under that relationship.
Mandatory membership implies a cardinality of at least one. Optional membership implies that the cardinality may be zero.
Fully Optional Relationship Membership
A relationship under which entities of both entity types can exist without participating in some pairing.
When both relationship memberships are optional (e.g., PUBLISHER commissions BOOK, where PUBLISHERs exist who do not commission any BOOKs, and BOOKs are written that have not been commissioned by a PUBLISHER), the relationship is called fully optional.
Most relationships are partly optional (i.e., only one of the relationship memberships has the property of optional).
Transient Relationship Membership
The membership of an entity of a specific type that may change between groupings under a specific relationship several times during that entity's life.
Exclusive Relationships
Two or more relationship memberships in which each entity may participate in any one, but not more than one, grouping under these relationships.
Exclusive Relationships and Integrity Conditions
If, at any time, a pairing holds between two entity types, it occasionally excludes the possibility of one or both of the entity types having pairings with other entity types at that time. In other words, the possibility of pairing between two entity types may depend on the lack of a pairing between one or the other or both of them and some other entity types.
For example, a PAYMENT may pay for one or more BOOKs or one or more MACHINEs but not for a mixture of both. Thus, the "PAYMENT pays for BOOKs" relationship membership and the "PAYMENT pays for MACHINEs" relationship membership are mutually exclusive. We call these exclusive relationship memberships.
Integrity Conditions
Exclusive relationships must be implemented through data integrity conditions. Exclusivity can be affected by an integrity condition, which can be complex.
A simple condition for the above example may be:
* If Payment Method = cash, the PAYMENT must be for BOOKs (because the company will never pay for machinery by cash)
A more complex condition may be:
* If Payment Method = cash and PAYMENT is sent to AUTHOR, PAYMENT pays for one or more BOOKs
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