About

aggregation is a containment relationship where one or more entities are part of a container entity.

In other word, aggregation is combining multiple pieces of data into one unit.

The container entity may be:

• another entity (for instance, a class that has student)
• or a nominal attribute (a tag, label or class) (for instance, a color that is present on sweaters)

aggregation is also known as:

• whole/part relationship
• catalog containment because when the container is destroyed, the content object is not destroyed (whereas in a composition it is).

In other term, this is not a dependency relationship (whereas a composition is)

The container is said to be an aggregate/compound type.

The structure to implements this kind of relationship is known as a collection.

Articles Related

Example

• Library and Students. Because the student can exist without library, the relation between student and library is aggregation.
• professor and students: when the professor dies the students do not die along with him or her.

Property

A aggregation is:

• a binary relation
• asymmetric - only one end of the relation can be an aggregation.
• transitive - if a aggregates b, if b aggregates c, then a aggregates c

Visualization

In a Object - Class Diagram (UML), an aggregation is represented with an association (ie the link) with a hollow diamond at the aggregate end.

Magical Number Seven, Plus or Minus Two

Nominal - Magical Number Seven, Plus or Minus Two

Operations

See Function - (Aggregate | Aggregation)