exclusive relationship

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Exclusive Relationship.
An exclusive relationship is one in which participation in one relationship excludes participation in other relationships. Such relationships are said to be part of an exclusive relationship group. All relationships in an exclusive group must have the same subject entity and the same optionality. Two types of relationship group exist: A mandatory exclusive relationship; in which one of the relationships must apply. And, an optional exclusive relationship; in which both of the relationships are optional. In both cases the participation in any one relationship of the group excludes participation in any other. In this example an employee may be either a manager or an operative, but not both.

Exclusion Arcs.
The exclusive relationship group is shown on the data model by an exclusion arc. The arc is drawn so as to cut the relationship lines stemming from the subject entity. It is important that relationships that are not part of the exclusive group are not cut by the arc. An arc may be split where it is not possible to place all components of a relationship group next to one another. Where an arc is split the 'mini-arcs' should be labeled with a common letter. This identifies mini-arcs as belonging to a specific exclusive group. In this example, assume that the entities of the exclusive group could not be placed adjacently - due to other relationships that are not shown. Here labels are used to relate the mini-arcs. If an entity participates in more than one exclusive relationship group then it will also be necessary to label the exclusion arcs and to refer to the appropriate entity description for an explanation. This diagram shows that an employee can be a manager or an operative. This exclusive group does not affect the other relationships - which show that employee could be involved in sales and/or administration.

Recursive Relationship.
This example of a recursive relationship shows a hierarchic relationship in which the entity 'manager' has a one-to-many relationship with itself.
In this example, “a manager may be responsible for one or more managers and may be responsible to one and only one manager.”
Both ends of the relationship are optional. This allows the senior managers not to report upwards and the junior ones not to be responsible for others.
However, this relationship would be better shown, with a mandatory end, by introducing the entity 'director'.
“Each Manager must be either responsible to one and only one director or to one and only one manager”.
When you have drawn your own solution for this, go to the next page to see the suggested solution.

This diagram communicates that each manager must be either:
Responsible to one and only one director. Or, Responsible to one and only one manager.
It also communicates that: “Each manager may be responsible for one or more managers.” and that “Each director may be responsible for one or more managers.”

Network Relationships.
A network relationship is one in which the entity has a many-to-many relationship with itself.
Consider the network relationship shown. A manufacturing plant has numerous components and sub-assemblies making up further units until the finished product results. A sub-assembly may be made up of other smaller sub-assemblies and may itself be used in a larger sub-assembly.
Both relationship ends must be optional as basic sub-assemblies may not have components, whilst others will be the final product.
Remodel this relationship using a new link entity 'standard component'. Note that each sub-assembly may be used as one or more standard components'.
When you have drawn your own solution for this, go to the next page to see the suggested solution.

The new link entity 'standard component' becomes the detail entity of both of the relationships. This diagram communicates the following:
1. Each sub-assembly may be used as one or more standard-components.
2. Each standard-component must be for the use of only one sub-assembly.
3. Each sub-assembly may be made up of one or more standard-components.
4. Each standard-component must be on the parts list of only one sub-assembly.

This content is an extract from GetAhead in Business Analysis - the FULL course is in eBook and multimedia CD-Rom format. TOP